I still remember the disappointment in 2002 when it became clear that humans had no more than 25,000 genes, about as many genes as a mouse! And with this very limited number of genes, we shall in addition to everything else, also be ready to defend ourselves against any possible unknown intruder?
Immune cells must respond to a whole range of threats that we have not yet experienced. It is impossible to know in advance what is to come, except that it is different from the body itself. The possibilities are almost endless, so it is quite unlikely that a single specific event we could have been prepared for upfront, is actually going to happen.
What else is there that is infinitely unlikely, and yet sometimes occasionally may be achieved? Correctly guessed: winning in Lotto. Instead of equipping the body with a large number of immune-genes that maybe, but only maybe, will come in handy, our bodies have developed a kind of betting company for immune genes. The betting ompany offers betting games (VDJ recombination) both for T cells and B cells and the basic rules are essentially similar:
Based on pure chance every “wannabe» B- or T-cell make a receptor that will be attached to the cell membrane on the outside of the cell, and which consists of two parts. Each part is similar to a lottery coupon where four genetic elements (V, D, J and C) are drawn from a limited collection of similar genetic elements. These are then assembled with a few random “additional numbers”. Taken together this will give a very large number of different receptors of the same kind. But every immune cell only get two lottery tickets and hence one receptor each. T cells and B cells do not play with the same lottery coupons, which ensures that their receptors can fill different functions.
Immune receptors on B and T-cells have a structure that allows them to bind to other molecules more or less strongly. If the bond is very strong, it represents the start of an immune response that aims to remove what triggered the response. We would prefer not remove the body itself, so mechanisms to remove immune cells with immune receptors that bind strongly to the body’s own constituents is needed. How this happens, I’ll come back to on on the blog at a later date.
Blog by Anne Spurkland published 14th November 2015
First published in Norwegian, 09/18/12