Autoimmune disease and chimeric cells
There are some very odd things that happen occasionally and are contrary to what we humans would like to believe is the natural order of things. One of the strangest notions I have come across in the biological world is the formation of microchimeras (pronounced 'micro-ky-meras'), meaning quite literally 'an animal with parts made from many animals'. During pregnancy, cells can roam between bodies. Fetal cells can transplant themselves into the mother, maternal cells can be found in the fetus, and cells from twins can swap places with each other. This doesn't happen all the time, but it seems to happen sufficiently often to warrant investigation into the clinical significance of this very bizarre, "Frankenstein-ish" phenomenon.
So, is microchimerism a good thing or a bad thing?
Microchimeras and autoimmune diseases Bad news first. A number of studies have found a correlation between the presence of these cellular trespassers and a group of conditions known as autoimmune diseases in which the immune system appears to turn upon the bodies' own tissues. Usually one particular organ or tissue is targeted leading to diseases such as diabetes, which is caused by the destruction of insulin-producing cells in the pancreas, or rheumatoid arthritis, in which the immune system attacks our joints.
Under normal circumstances the immune system is programmed to live peacefully with cells from its own body and to destroy only foreign invaders such as germs or cells with the wrong DNA.
Cells containing different DNA can arise through mutation, the mechanism which produces cancer, through organ transplantation, including bone marrow transplantation, or through microchimerism. Such cells express a slightly altered pattern of chemical markers on their surfaces, helping the immune system to identify them as foreign and leading to the mounting of an immune attack which destroys them.
For reasons which we don't fully understand, occasionally the immune system mistakes our own healthy tissue for that of an invader and unleashes the full power of our defences against our own body producing an autoimmune disease. But recently scientists announced that they have found microchimeric cells in the diseased tissues of women with some types of autoimmunity, leading them to speculate that the immune system is actually working just as it should. Indeed, perhaps cells from a woman's child(ren), which have become lodged in the affected tissues, are the ones that are attacked rather than the mother's own cells, creating, in effect, cases of transplant rejection. In the ensuing battle, our own healthy tissue, surrounding the trespassing cells, becomes damaged by 'friendly fire'. The evidence is very tenuous at this point and is only one of many possible explanations for the aetiology of certain types of autoimmune diseases, but it does offer one explanation for the observation that some of these diseases are commoner in women than men, and are related to pregnancy.
Potential benefits of microchimerism Luckily it's not all bad news because there can be positive aspects to microchimerism too. Many maternal immune systems are quite happy to tolerate their offsprings' cells (practice perhaps for tolerating their whining?). In cases of a microchimeric mother requiring a transplant from a child that wouldn't otherwise be a good match, there is evidence for improved transplant acceptance rates. Perhaps the immune system has been tricked through years of harbouring the child's cells (containing alien DNA) into thinking that the new organ is a naturally-occuring part of the body ?
From this we learn that microchimerism can be good or bad, though perhaps most times it won't make any difference at all. However, it brings us to an important consideration of what we are made of. Our environment has long been known to play a significant part in our development as people. Perhaps more crucial is the interaction between our genes and the environment in which we live. Whether that be how close we live to a pylon, or how dysfunctional our families are, the net result impacts greatly upon our future health.
At one time scientists believed that two people with the same DNA (e.g. identical twins) possessed the same genetic potential, even if they were living in different environments. Microchimerism, however, casts doubt on these traditional views. Despite everything we learned in A level biology, it seems that not every cell in our body must arise from that crucial moment of conception and that one part of our body may have a genetic inheritance arising, at least in part, from another human being. If many of us are chimeras, it puts a new twist on the cloning issue - copying DNA from one cell would not yield a replica of the cloned individual even if they were somehow nurtured in identical environments. Humans, and the intrigues of the biological world, are much more complex than that.