Everyone knows that bacteria/viruses/pathogens make you sick.
Everyone knows your immune system fights back against these guys so you get better.
But did you know that pathogens can (accidentally) make your immune system mutiny? Turn your own immune system against you?
They can! Through ‘molecular mimicry‘!
Lets say you woke up this morning, your throats super sore, huge tonsils– crap. Strep throat. Strep is usually caused by group A streptococcus bacteria. Normally your immune cells trot about, exchanging information, tips on where the little buggers are hiding, which epitopes are working the best, etc.
Mutation– The B-cells cells that react the best to Strep proliferate and change, just a little bit, through somatic hypermutation. Teeny-tiny changes in an antibody leads to lots of lots of antibodies that recognize the bacteria in teeny-tiny different ways. A theme in variations.
Natural Selection– So you now have lots and lots of B-cells that make an antibody that recognizes Strep. But maybe B-cell 6, 49, and 572 produce antibodies that are awesome at kicking that bacterias butt, while all the others are ‘just okay’. Well the 6, 49, and 572 B-cells are competing for survival signals with the just okay’ B-cells. The ones that react the best get the signal to survive, and they proliferate more and change just a tiny bit more. The ones that are ‘just okay’ die off.
This is a really cool process, but it has draw backs.
You might accidentally generate an antibody that works great against Strep… but also works great against your friggin brain!!! AHHH. Antibodies that stick to ‘self’ cells can lead to activation of the complement system (your self cells explode), or tell other immune cells to eat your self cells, or the antibodies might actually mimic a chemical signal and activate receptors on your cells in a weird way!!
To go back to the Strep example, a complication of strep throat is rheumatic fever. RH is caused by these self-reacting antibodies attacking your joints/heart/etc. One group has identified an antibody that mimics gangliosides, leading to too active CaM Kinase II, and basal ganglia death in your brain: Sydenham chorea.
The Strep isnt doing this on purpose. There is no benefit to staph for giving you jerky face movements.
Your immune system isnt doing this on purpose. There are many, many check-points in the immune system developmental process to make sure nothing that reacts to self can get through.
But more and more diseases are being linked to molecular mimicry– this same group found that antibodies that react to a different portion of Staph mimic epinephrine (!!!) and bind to epinephrine receptors on your heart, leading to a kind of heart disease.
Other people have been trying to connect molecular mimicry to lupus, multiple sclerosis, type I diabetes– any disease where your immune system is destroying the very things its supposed to protect. Kinda scary to think that the cold you got when you were 5 could make you have arthritis when youre 50…