Wednesday, May 25, 2016

Don’t Be So Sensitive!

Biology Concepts – immune hypersensitivity, allergy, autoimmune disease

Stone Mountain in Georgia is one big hunk of
granite. There is a bas-relief carving of Jefferson
Davis, Robert E. Lee, and Stonewall Jackson that
covers three full acres of space! Stonewall Jackson
is on the far right. Stonewall immortalized on a
stone wall, interesting…. but didn’t their side lose?

C.S. Lewis was a 20th century British writer who penned the Chronicles of Narnia books. Thomas “Stonewall” Jackson was a brilliant general in the Army of the Confederacy during the American Civil War. Can you name something these men had in common, but wish they didn’t?

---– They were both shot by their own troops during battle. It wasn’t on purpose; Lewis was wounded by a British shell that didn’t have enough oomph to get over the British’s own lines during World War I. One piece of metal lodged deep in his chest and could not be safely removed. It remained near to his heart until 1944.

During the Battle of Chancellorsville in 1863, Stonewall Jackson led a night reconnaissance mission that was mistaken for Union scouts. A confederate patrol fired on Jackson as he looked over the Northern lines from horseback. His left arm was amputated in an effort to save his life, but he died of pneumonia eight days later.

These were incidents of “friendly fire,” in which people meant to help you fend off the enemy end up hurting you. Too often, incidents of friendly fire take place in your body as well. In biology, these are called immune injuries and they can be dangerous exceptions. The immune system is designed to help the body fight off foreign invaders and dangerous molecules, but there are those instances when its actions harm the host.

Allergies are a good example of immune reactions gone wrong. Originally (1906) meant to denote any immune injury, we now we look at allergic reactions as immune responses to non-pathogenic, and in many cases, non-harmful antigens. Who could be harmed by a peanut, except for those allergic to it.

Peanut allergy is nothing to take lightly. It is
estimated that 3 million people now react to
peanuts, even to foods prepared in kitchens
where there are peanuts. This is an immediate
anaphylaxis response, with inflammation and
often respiratory distress.
A person can have an allergic reaction to an aeroallergen – something carried in the air, like dust, pollen, or pet dander. Food allergies occur generally with milk, wheat, peanut, or egg; many of these dissipate as children mature. Drug allergies can develop when small molecule pharmaceuticals break down, combine with host proteins or cells, and are then recognized as foreign. Some people are allergic to some venoms, like bee venom; their reactions can go beyond the pain of the sting.

In allergic reactions (atopic reactions – atopy is from Greek for “out of place”), there is first a sensitizing dose, wherein your body develops a hypersensitivity to the allergen. This is when your body builds an immunologic memory for the antigen, like we talked about a few weeks ago. Any exposure to the allergen after this brings a stronger response.

The exception to this sensitizing dose idea is when a new allergen looks like another allergen, ie. cross reactivity. Many latex allergies do not seem to have a sensitizing dose, but the patients also happen to have an allergy to banana, kiwi, or avocado. This is called the latex-fruit syndrome…catchy name, isn’t it?

Allergic reactions can occur just where the allergen contacts the immune system, like itchy hives (urticaria) for contact dermatitis, or a runny nose for pollens grains that are breathed in. Sometimes the hypersensitivity goes further and there is a life threatening reaction. We should describe the different kinds of hypersensitivity so you can diagnose your friends at parties.

Type I hypersensitivity is an immediate reaction, with symptoms lasting for a short time. Sometimes there is a more chronic response, especially if the antigen sticks around. Type I reactions are the allergies we all know and hate. The term for the reaction is scary, “anaphylaxis” (ana = exceedingly, and phylaxis = guarding), but it isn’t always life threatening.

In type I hypersensitivity, the allergen is recognized by specific IgE antibodies. Antibodies come in several flavors, including IgG (circulating antibody), IgM (antibody as cell receptors for first encounters), and IgA (in saliva and tears, etc.). IgE immunoglobulins are present in the tissues or on the surface of certain immune cells from some previous, sensitizing dose. The antibody has a variable end that recognizes the antigen and a constant end (Fc) which is recognized by other immune cells. When two or more IgE antibodies bind to the antigen (called crosslinking) and the Fc portion attaches to a mast cell or basophil, these immune cells will release their contents.

On the left is an electron micrograph of a mast cell,
an innate immune cell that mediates allergic responses.
On the right, you can see the granules inside the mast
cell that contain histamine, bradykinin, and other mediators.
When IgE and an antigen crosslink on the surface of the
cell, the granules release their contents into the
extracellular space.
Mast cells contain histamine, which causes blood vessels to dilate, airway smooth muscle to contract, itching, and stomach acid secretion. Mast cells also have bradykinin that increases mucous production, as well as other chemicals. Mast cell degranulation (release of internal granules containing the histamine, etc.) makes your eyes water, your skin get hot and itch, makes it harder for you to breathe, and might produce hives on your skin.

The reaction might remain local, but if it triggers the same reaction throughout your circulatory system, it can cause anaphylactic shock, a true medical emergency characterized by low blood pressure and respiratory difficulty. It can and will kill you if not treated immediately. And all this because some innocuous small molecule and an IgE antibody caused your immune system to over react!

Type II hypersensitivity reactions are also mediated by antibodies (IgM or IgG type). The triggering antigen might be some foreign molecule bound to a host cell or even an antigen on your own cells that your body has mistaken for foreign. In the case of penicillin allergy, the drug becomes bound to your cells; this complex triggers the immune response. If the antibodies are directed toward your cells or mistake your cells as foreign, this is called an autoimmune reaction. Examples could be systemic lupus erythematosus (SLE), some type I diabetes, or Hashimoto’s thyroiditis.

In some type II reactions, the antibodies that bind to the antigens trigger the complement system in your tissues to activate. Complement is part of your innate immune system that ends up marking cells for destruction by phagocytosis, or destroys them itself by punching holes in the target cells. In some cases, the antibodies bound to the cell trigger innate immune cells called natural killer lymphocytes – you can guess what they do to the target cell. I guess everyone is a natural born killer on the inside.

Natural killer cells are lymphocytes, but are part
of the innate immune system. These two are
attacking a cancer cell (red). Natural killers
specialize in killing cancer cells and virus-infected
cells. Natural killers are unique in that they can
recognize stressed cells in the absence of binding
The last type of immediate hypersensitivity is type III. The danger of this type of reaction comes from masses of antigens surrounded by antibodies. When these immune complexes (also called Ag-Ab complexes) become large, they can get stuck in tight places and bring an inflammatory response. Examples of immune complex diseases are autoimmune rheumatoid arthritis, some types of glomerulonephritis (inflammation of the filtering units of the kidney), and SLE also triggers this response.

Type IV hypersensitivity is the exception; this response can take several hours to develop and is the only hypersensitivity reaction that does not involve antibodies. Lymphocytes of the adaptive immune system interact with the antigen (be it foreign or domestic) and release many chemical mediators, called cytokines, that mediate immune and inflammatory reactions. Allergic contact dermatitis from poison ivy is a common, but relatively benign, example of this type of hypersensitivity.

Most hypersensitivities are reactions to things that shouldn’t have been problems in the first place. Allergies are just the most common manifestation of immune hypersensitivity. I don’t have them to any degree, but I see the havoc they wreak on my wife and our son. He is so allergic to wool that he breaks out when he counts sheep in bed!

But even allergies might have a hidden benefit. A study in 2008 indicated that people with allergies actually have a 25% less chance of developing a certain type of immune cell cancer, called B-cell non-Hodgkin’s lymphoma (NHL). If that person has three different allergies, they are 40% less likely to develop NHL.

This seems amazing, but it is supported by a 2011 study showing that people with allergies are 25% less likely to develop a type of brain tumor called a glioma. Glial cells protect and support the neurons in the brain; abnormal growth of these cells can lead to pressure and death of brain cells. Still think allergies are annoying?

Sneezes leave your mouth at over 100 miles and
hour and can spread droplets over 30 feet. Sneezes
may help get ride of unwanted antigens, but other
people don’t want them either, so cover your mouth.
Sneezing into the crook of your elbow is best for
limiting spray and contamination – I saw it on
Researchers don’t know the reason for this benefit yet, but hypotheses include that allergic reactions (watery eyes, sneezing, runny nose) help to eliminate potentially carcinogenic pollutants from our bodies, or that allergies stimulate the immune system and make it better at detecting and destroying cancer cells.

Learning that allergies might prevent cancer may make you less likely to take that antihistamine capsule. In fact, the treatment for all immune hypersensitivity reactions involve avoiding the molecule, removing the offending antigen and antibodies, and/or suppressing the immune system. We take corticosteroids, antihistamines, and other drugs to prevent the actions that might be saving us from cancer. However, you can help protect yourself without drugs as well—just catch a parasitic infection.

Parasitic worm infections, whip worm (Trichuris trichiura) or schistosoma for example, have a tendency to dampen the immune response, and can prevent some relapses in autoimmune diseases such as multiple sclerosis. A 2005 study indicates that some success has been had after dosing Crohn’s disease patient’s with intestinal worms.

Meet Pediculus humanus capitis, the common head
louse magnified only 80x. It is an ectoparasite,
meaning it lives on the host, not in the host. They
have been around for a long time; they have been
found on Egyptian mummies. This is why most
Egyptians shaved their heads and wore wigs.
For those of us without life-threatening autoimmune disorders, a 2009 study suggests that Pediculus humanus capitis infestations (head lice) can dampen the immune system enough to prevent allergies and some asthma attacks. Your choice - but don’t let anyone borrow your comb!

Parasites seem to have evolved specific mechanisms that inhibit the reactions that would eliminate them from the host, so they dampen immune responses as a defense. The mechanisms have not been worked out and may be parasite specific. Even malarial and leishmaniasis parasites can suppress the immune response, but I don’t recommend that you contract a deadly infection just to alleviate your allergies.

These last studies suggest that we may be living too cleanly – let’s take a look at that next week.

Calboli FC, Cox DG, Buring JE, Gaziano JM, Ma J, Stampfer M, Willett WC, Tworoger SS, Hunter DJ, Camargo CA Jr, Michaud DS. (2011). Prediagnostic plasma IgE levels and risk of adult glioma in four prospective cohort studies. J Natl Cancer Inst. DOI: 10.1093/jnci/djr361

Joseph A Jackson, Ida M Friberg, Luke Bolch, Ann Lowe, Catriona Ralli, Philip D Harris, Jerzy M Behnke, Janette E Bradley (2009). Immunomodulatory parasites and toll-like receptor-mediated tumour necrosis factor alpha responsiveness in wild mammals BMC Biology DOI: 10.1186/1741-7007-7-16

For more information or classroom activities, see:

Allergy –

Immune hypersensitivity –

Autoimmune diseases –

Wednesday, May 18, 2016

Ironing Out The Black Death

Biology concepts – iron, genetic disease, infectious disease, immune evasion

It is strange to think of people as rusting, but there are 
days when I get up and swear that my joints have 
frozen – my age makes me assume it is rust. 
In truth the molecules of rust are very much like 
some molecules in your body; too many of these in 
the wrong places, and maybe you are rusting.

Believe it or not, someone you know is rusting - and it probably saved his/her ancestor’s life.

Animals require iron to survive; normal adult humans carry about 3.5-4 grams of iron in their bodies. It’s vital for every cell. Red blood cells use iron as part of the hemoglobin molecule that carries oxygen, But all other cells use iron in part of electron transport chain that makes ATP, and in the synthesis of DNA.

In plants, iron is used in chlororphyll production, in nitrogen fixation, and in regulation of transpiration (moving water and nutrients up to the leaves). Plants are a decent source of dietary iron, but heme iron (from meat) is much more easily absorbed.

In both plants and animals, the amount of iron is highly regulated. Iron is most often bound to proteins; one type in cells, another in the blood, and they lock it up tight. When you need more, your gut cells (enterocytes) release some of their stored iron and then take in more from the food you eat.

People who absorb too little iron (from poor diet or absorption defects) have a hard time carrying oxygen to their tissues because they don’t have enough hemoglobin. They are fatigued, dizzy, lose their hair, and less able to fight off infections. Weirdly, they may demonstrate pagophagia; a compulsion to eat ice! The reason for this is open for discussion, but one hypothesis says there is an ancient crunching desire, related to chewing on bones to get at the iron-rich marrow.

Pagophagia (eating ice) is one type of pica. In pica, a
person craves to eat something that is not a food source.
Some people with pica will eat hair (trichophagia)
or dirt (geophagy). I guess if you have to have pica,
ice craving isn’t so bad. And yes, some people crave
plastic, like parts of your keyboard.

Too little iron keeps you sick - and apparently always refilling the ice tray. But too much iron is just as bad; both ends of the scale can kill you.

Hereditary hemochromatosis (HH) is an autosomal recessive (need two mutated copies) disease of iron storage and transport.  Patients with this disease may have as much as 20-40 grams of iron in their bodies; they can even set off metal detectors at airports!

All this iron causes medical problems too. People with HH will accumulate iron in their liver, heart, skin and other tissues. Excess iron plus fats can produce free radicals and oxygen radicals. The radicals can react with many molecules, including those you need in order to keep your cells functioning properly.

Radicals can break down enzymes, destroy mitochondria, and even react with the iron itself to produce iron oxide – rust; biological rust being called hemosiderin. Could HH patients be like the frozen Tin Man that Dorothy finds in the Wizard of Oz? Of course not, tin doesn’t rust – it’s a good thing L. Frank Baum was a writer and not a metallurgist!

The brown color is hemosiderin pigment that has been
deposited in the tissues.  Most times, your body will
resorb this colored material, like when a bruise goes
away over time. In hemochromatosis, there is too
much hemosiderin to be completely removed.

Over time, the damage from free radicals and from hemosiderin buildup causes systems to shut down. Without treatment HH is lethal - so it is important to know how all that iron gets there.

We said above that enterocytes are the storage area for iron absorbed from your diet. In HH, the export signal is broken and they keep dumping their stored iron into the bloodstream. Even worse, the enterocytes lose the ability to sense if the body needs more iron. As a result of HH, gut cells keep absorbing more iron and releasing it into the bloodstream.

It’s a bad thing to inherit hemochromatosis…..EXCEPT if Yersinia pestis is lurking in the environment. Y. pestis is the bacterium that causes the plague. The organism can be passed from person to person, but also from fleas to people, and from fleas to animals to people.

You can read about how Y. pestis ensures it is transmitted to a new host from the flea’s midgut, but for reasons of decorum, I won’t go into it here. And I suggest you don’t eat before you read about it.

Y. pestis plague comes in three flavors; septicemic (travels through the blood), bubonic (causing swellings), and pneumonic (some organisms go to the lungs). In the case of pneumonic plague, coughing promotes transmission from person to person and is more lethal. But bubonic plague is more painful.

The plague has been a killer throughout human history, but Y. pestis’ relationship to the flea is evolutionary rather new. About 20,000 years ago, Yersinia killed the flea as well. According to new research, it took relatively few genetic changes to allow plague bacteria to keep the flea alive and to survive in its midgut. It was at this point that humans' trouble really began. It is estimated that a third of the population of Europe was lost to plague in 14th century. The infection still occurs today, but is highly treatable with antibiotics. Your immune system has problems getting rid of Y. pestis on its own.

Normally, your immune system recognizes foreign organisms and eliminates them, through either innate or adaptive mechanisms. However, Y. pestis has several tricks up it sleeve to avoid recognition and destruction by your immune system.  

The lymphatic system is comprised of vessels, and
is considered part of your circulatory system. It
helps in eliminating wastes from the blood and
tissues, aids in absorbing fats and fat soluble
vitamins, and regulates fluid levels. A main function
is to move fluid and cells through the checkpoints,
the lymph nodes. Here, the fluid is checked for
foreign molecules and antigen presentation to the
immune cells in the nodes.

Immune cells can circulate in your blood, move in and out of your tissues, or may be located in your lymphatic system. In the lymph nodes, they gather to exchange information, like workers gossiping around the water cooler. If an antigen processing immune cell (APC) has encountered a foreign antigen, the APC will break it down and place pieces of the antigen on its surface, so the antigen can stimulate other immune cells.

The processed antigen is presented to the many types of immune cells in and moving through the lymph nodes, including B cells that make antibodies, and T cells that direct immune responses or directly kill organisms. This quickly increases an immune response; one cell encounters the invader, but by going to a central location (lymph node), thousands of cells can be stimulated.

Amazingly, Y. pestis actually lives and reproduces in your lymph nodes! The painful swellings in bubonic plague are the inflamed lymph nodes where the organism is reproducing. Each swollen node is called a buboe, hence the name of the plague. Buboes occur most commonly in the armpit (axilla), on the neck, or in the groin area – not a pleasant way to spend a weekend - maybe your last weekend.

The lymph nodes are the headquarters for stimulating immune responses, yet the Y. pestis lives here very happily. It manages this through several evasion mechanisms:

1)   antiphagocytic proteinsY. pestis can inject proteins into phagocytic cells that makes them poor at eating and killing. These proteins also makes immune cells unable to signal other immune cells that Y. pestis is there.
2)   invasion proteins – plague bacteria can avoid immune detection by living inside several different host cell types; the macrophage is the major example.
3)   survival proteinsY. pestis  can live inside the macrophages that are supposed to destroy them by turning off macrophage killing mechanisms.
4)   heme stealing proteinsY. pestis can steal iron from the host. And here is where HH comes in.

Here is a buboe on a plague patient’s neck. It is not unlike the parotid 
salivary gland swelling that takes place during the mumps, just
bigger, more painful, and more lethal. I chose to show one from the
neck precisely because I didn’t want to show you one from the groin.

Here is an organism that is perfectly happy living inside and in the company of the cells that are supposed to kill it - we’re doomed. Yet having a disease like hemochromatosis can save us. How can that be? Well, microorganisms need iron too. For much the same reasons as animals and plants, bacteria and other microorganisms must have a supply of iron. They may get it from their diet, or, as is the case with Y. pestis, they steal it from their host.

I can hear what you're saying - this doesn’t seem to make sense since HH results in lots of iron in cells. True, but there is an exception. HH leaves two cell types starved for iron - the enterocyte, which we already know about, and the macrophage. The reason for iron-poor macrophages during hemochromatosis is not completely understood, but one possibility is that the HH mutation affects macrophages the same way it affects enterocytes.

One important function of macrophages is to eat and destroy old host cells, including erythrocytes. The iron of the hemoglobin from all those degraded RBC’s is stored and recycled; this is an important mechanism that the body uses to reuse the iron it already has. But in HH, the macrophages may be pumping out the iron they take up from old RBCs, just as the enterocytes keep pumping out the iron they take up from the gut contents.

The iron-poor macrophage essentially starves the intracellular plague bacteria by not providing them with iron. This is a happy accident for us, but it isn’t as if the macrophage doesn’t already know this trick. Iron can be an important immune weapon. In mycobacterial infections (that cause pneumonia), macrophages actually raise the iron concentration in the ingested bacteria and kill them that way. In other infections, macrophages sequester their iron and starve the organisms.

Bloodletting is an old time treatment for nearly every
disease. They thought that disease was caused by too
much blood. Strange, but bleeding (phlebotomy) is now
the accepted treatment for hemochromatosis. Leeches
are now used as anti-clotting mechanisms, and fly
maggots are used to clean out dead tissue – all are
gross, and all are effective!

Macrophage iron manipulation is not a natural immune response to Y. pestis, but HH helps to bring about the same effect, and this makes HH valuable. It is believed that many survivors of the plague in the 12th through 15th centuries had hemochromatosis. What is more, the gene is present in as many as 1/3 of living people of European descent, meaning that HH is probably massively underdiagnosed. It is likely that you know someone with HH, whether they not it or not.

Natural selection kept this mutation in the gene pool because it presented a reproductive advantage in times of plague. With antibiotics, we probably do not need this mutation any longer, but it is here and will take quite a while to be bred out of the population, especially since HH treatments (like bleeding, see the picture at right) help people live with the disease long enough to pass on their genes.

There are more examples of bad genes saving us from disease, like chemokine receptor mutations preventing HIV infection and aldehyde dehydrogenase mutations discouraging alcoholism. But next week we will focus on immune systems run amok and how parasites can reel them in.

Chouikha I, Hinnebusch BJ. (2012). Yersinia-flea interactions and the evolution of the arthropod-borne transmission route of plague. Curr Opin Microbiol. DOI: 10.1016/j.mib.2012.02.003

For more information or classroom activities, see Survival of the Sickest, by Dr. Sharon Moalem, or the following sites:

Iron in biochemistry –

Hereditary hemochromatosis –

Y. pestis plague –

Immune evasion strategies –

Wednesday, May 11, 2016

Viva La Evolution

Biology concepts – evolution, reproductive advantage, natural selection, co-dominance, X-linked genes

Last week we learned how less aggressive strains of malaria were used to treat neurosyphilis and how they may be useful in treating HIV infection. This week, we will turn 180˚ and see if other diseases can help prevent or lessen the effects of malaria. In the process, much can be learned about natural selection and reproductive advantage.

Plasmodium-infected red blood cells develop knobs,
the surface protrusions seen on the left erythrocyte.
These knobs are covered in a certain protein that
inhibits the immune system’s ability to recognize this
cell as infected and respond to it. The cell on the right
is also infected with P. falciparum, but has a mutation
that prevents knob formation. Image credit: Ross
Waller and Alan Cowman.
As you undoubtedly remember from last week, malaria is a parasite-caused infectious disease that is transmitted from human to human by mosquitoes. The parasite, Plasmodium falciparum, takes up residence in the red blood cells (RBC) to reproduce. The red cells burst to release the organisms, and this brings fever and weakness.

As far back as the 15th and 16th centuries, quinine, made from the bark of the cinchona tree, was being used in Peru to treat malaria. Chloroquine, mefloquine, and quinine all work against malaria in similar fashion. Because of their neutral pH, they move across membranes easily including the lysosome membrane. Once inside the lysosome, they become charged and can’t get out. This includes the trophozoite-containing lysosomes. In the RBC, trophozoites consume hemoglobin to obtain amino acids, and the heme is digested in the lysosomes to form a black malaria pigment. The quinine drugs in the lysosome bind up the heme and produce a toxic product (cytotoxic heme) that kills the parasite.

There are other classes of drugs that are useful against P. falciparum. Primaquine and the artemisinin drug, artesunate, act by a completely different mechanism from that the quinine drugs. Artesunate is excellent for treating P. falciparum malaria, while primaquine is often used in conjunction with quinine to treat P. vivax or P. ovale forms of the disease.

These drugs work by breaking down – weird, but this is how many drugs work. It isn’t what you swallow that kills the organism, it's the metabolites (the products made by your biochemistry breaking down the drug) that are active. In the case of artesunate and primaquine, the heme molecule in the red blood cells releases peroxide from the parent compound (the drug you take). This is just like the peroxide you use to wipe out cut in order to prevent infection.

Artusenate comes from the sweet wormwood
plant. Chinese herbal medicine has used it for
thousands of years. A recipe for an Artemisia
based malaria medicine was found on a tablet
from the Han Dynasty (206 BCE to 20 CE). It is
now being investigated as a treatment for breast
cancer, also based on its ability to form radicals.
Oxygen is crucial for cellular function because it can gain electrons and can react with many other atoms. Unfortunately, this also makes it harmful to your cells as well. Without proper supervision, forms of oxygen that have picked up an extra electron or two (peroxide, superoxide, nitric oxide) can react with many important molecules in your cells and leave the cell impossibly damaged.

The cell has defenses against free radical damage, but higher than normal concentrations render the RBC fragile; on the tipping point of destruction. Treatment with primaquine or artesunate makes the cell inhospitable for the parasite, the red blood cells become flop houses instead of five star hotels. The parasite’s operating instructions are to survive and reproduce, but these drugs pull up the erythrocyte welcome mat and the parasite seeks moves on to seek friendlier accommodations.

Unfortunately, some strains of P. falciparum have become resistant to some quinine drugs, especially chloroquine. The free radical generating drugs are still useful, but scientists in Western Cambodia recently reported artesunate drug resistance there. The parasite has evolved – evolutionary pressure is everywhere. The actions of humans have put pressure on the organism to evolve; those parasites with mutations to resist the drugs have a reproductive advantage, and those mutations get passed on. We had better have something else on our plate to combat malaria – we're working on it, but nature has provided some help as well.

There are natural defenses against malaria. We have seen that a fragile red blood cell helps in preventing are lessening the disease course of malaria. What else might do that? This is where human genes come into play.

Sickle cell disease creates a very fragile RBC. The mutation is just a single DNA base change in the hemoglobin beta chain peptide, but the result is a hemoglobin molecule that becomes pointy and can tear the red blood cell apart, or can get stuck in small blood vessels and prevent good blood flow. Reduced blood flow starves the downstream tissues of oxygen.

You get one gene for hemoglobin beta chain from each parent. The disease comes when an individual receives mutated genes from both parents. But that doesn’t mean that sickle cell anemia is a recessive trait. If you have one copy of the mutated gene, then you will have sickling problems when oxygen concentrations are low, like during exercise or at high altitude.
Sickle cell disease or a sickle cell trait episode can result in red blood
cells clogging up vessels and organs. On the left is an absolutely
HUGE spleen from a sickle cell patient. On the right is a normal sized
spleen, about 20% the size of the injured spleen on the left. A normal 
spleen is about the size of your hand, maybe a little skinnier.

If sickle cell anemia was a recessive disease, then a single wild type (normal) gene would be dominant, and you would show no disease. Instead, sickle cell anemia is co-dominant, one mutated allele (copy of the gene) is like having half the disease; it only shows up in certain circumstances.

This can still be a pebble in your shoe, just ask Ryan Clark, the Pro-Bowl safety for the Pittsburgh Steelers. In a 2007 game in Denver (altitude 5300 ft, 1616 m), Ryan almost died from a sickling attack during the game, and ended up having his spleen and gall bladder removed (remember that sickled RBCs can clog blood vessels, especially in blood rich organs like the spleen).

When Pittsburgh next played Denver, Clark didn’t even make the trip. This just happened to be the 2011 playoff game in which Tim Tebow threw a long touchdown pass in overtime to the receiver being covered by Clark’s replacement. Sometimes disease can change how sports evolve as well.

Thalassemia is another example. This is a group of inherited disorders wherein there is reduced production of one of the subunits of hemoglobin (hemoglobin is made from 2 alpha and 2 beta subunits). Alpha-thalassemias have mutations in the alpha subunit; likewise for beta-thalassemia.

Reduced subunit number means reduced hemoglobin number; the blood won’t carry enough oxygen, and the patient is constantly oxygen-poor in his/her tissues. Having two mutated alpha genes is lethal in the very young (called hydrops fetalis), but you can live with one mutated alpha gene, one mutated beta gene, or even two mutated beta genes.

This the broad bean, or fava bean in opened pod
and out of the pod in a bowl. The ancient Greeks
used to vote with fava beans, a young white bean
meant yes, and old black one meant no.
Sickle cell trait (one mutated allele), and thalassemias result in fragile erythrocytes. This makes them poor hosts for malaria, and confer a resistance to the disease - bad genes aren’t bad in every case. And just for good measure here is another example.

Favism, better called glucose-6 phosphate dehydrogenase deficiency (G6PDH), is an X-linked genetic disease; the gene is on the X chromosome. A female (XX) has two copies, so having one mutant copy is no problem, but a male (XY) has only one, so getting a mutated copy from your mother means that you ONLY have the mutated gene – this brings the disease.

The enzyme G6PDH works in several pathways; in your red blood cells, it is the only source of reduced glutathione, an important antioxidant. This means that things that trigger free radical formation in your red blood cells will trigger the disease – lots of weakness and lack of energy. If there is enough erythrocyte destruction, one could die.

Triggers include broad beans (fava beans), hence the name favism. Other triggers include many drugs, including primaquine and artesunate, the anti-malaria drugs that induce free radicals. Having G6PDH-deficiency is like having your own artesunate pharmacy right in your cells - you naturally have higher oxygen radical levels in your RBCs, so the malarial parasite can't live there.

Not by accident, sickle cell mutation is more prevalent in people of Sub-Saharan African descent, thalassemia mutation is more common in people from the warm, moist Mediterranean, and G6PDH-deficiency is found most commonly in the Mediterranean and Southeast Asia. These just happen to be the areas where malaria-carrying mosquitoes are most abundant. Evolutionary biologists make the argument that natural selection has maintained these genes in the populations because they provide a reproductive advantage to the species.

Left image: dark green is where there is thalassemia and yellow and red are where there is sickle cell. Right image, light green is where there is favism, and inside the light blue outline is duffy antigen mutation. It is
interesting that these areas are also where malaria is endemic.

You might die from sickle cell disease, but probably not from sickle cell trait or beta-thalassemia. Learning not to eat fava beans makes the G6PDH mutation less lethal. One might very well live to an age where one could mate and pass on his/her genes. The diseases might still kill the patient, just not as soon as malaria would.

Malaria is a killer, and significantly, a killer of the young. In East Africa, children are bitten by the anopheles mosquito on average 50-80 times each month. They very well might not reach an age to reproduce. Therefore, having sickle cell trait, thalassemia, or favism provides a reproductive advantage in these environments and natural selection has resulted in these alleles remaining in the populations in these areas.

The Duffy antigen (DARC) is important for P. vivax
entrance into the red blood cell. The Duffy binding
protein (DBP) interacts with DARC, the yellow parts
of the DBP are variable, and can be used to bind an
antibody. These variable areas overlap the binding
site, and can be used to make a vaccine for P. vivax.
Evolution maintains some diseases in order to combat others. It isn’t by design, it's by biology; no big plan is involved, as exemplified by the Duffy antigen. All your cells have proteins on their surfaces. One, called DARC (Duffy Antigen Receptor for Chemokines, or Duffy antigen) helps your cells receive signals from your immune system. In those people with a specific single nucleotide polymorphism (SNP) for Duffy Ag, the antigen is not present on red blood cells (though it is still present on all other cells).

Since P. vivax uses Duffy Ag as a way to enter the red blood cells, those with the Duffy SNP are resistant to P. vivax malaria – they don’t even have to suffer with some other disease; just a simple case of chance.  And the prepared mind exploits chance – the Duffy antigen binding protein is now a candidate for use as a P. vivax vaccine.

Next week, how the plague was defeated by a genetic disease.

Chootong P, Panichakul T, Permmongkol C, Barnes SJ, Udomsangpetch R, et al. (2012). Characterization of Inhibitory Anti-Duffy Binding Protein II Immunity: Approach to Plasmodium vivax Vaccine Development in Thailand. PLoS ONE , 7 (4) DOI: 10.1371/journal.pone.0035769

For more information or classroom activities, see:

Malaria –

sickle cell mutation –

thalassemia –

favism –

duffy antigen –