Now that we understand just how important it is to recognize bad "bugs", how do our bodies become capable of recognizing the millions and billions and even trillions of different arrangements of things that can make us sick?
Well, it is through a Darwinian-style selection process where the fittest survive andthe weakest die. In our bodies we have cells that are specialized immune cells called T-cells. They go to "school" to learn the difference between "self" and "non-self".Certainly one would not want his/her own immune system to attack his/her own body!Sometimes this does happen and it is referred to as an autoimmune disease. However,normally, the T-cells are educated in a very tough school so that they do not attackone's own body or "self".
But how exactly are they trained to recognize the differencebetween self and non-self?Well, T-cells are capable of being able to tell the differencebetween certain molecules(antigens) that are presented(by MHC class I molecules) onthe surfaces of all "self" cells inthe body that they are supposedto protect compared to allforeign or "abnormal" antigensthat are associated with outsideinvaders or internal diseaseprocesses (like cancer). For