Our immune system is designed to protect us from any invader, microbes or larger parasites. To do so our body must be able to distinguish between his own materials and those that originate outside it. But knowing that all living creatures are made up of basically similar building blocks, the capacity of any creature to distinguish the molecules of which it is composed, from practically all others is quite remarkable: our white blood cells, or lymphocytes, mainly T cells (thymus) and B cells (bone marrow) are the cells in charge of the job.
Lymphocytes have structures called receptors on their outer surface The shape of the receptors is complementary to a pattern on the surface of the molecule that is to be recognized. These receptors act as keys that fit locks on molecules of invading organism. Every cell in the body carries bits of protein on its surface. Passing T lymphocytes sense these proteins and leave them alone.

The lymphocyte population of our body can recognize virtually any complex molecule that nature or science has devised. This very remarkable ability is achieved because each white cell develops its own specific antigen receptor.The total amount of different receptors on all of the lymphocytes cannot be measured, but it is estimated to be at least one million different receptors!
After a lymphocyte encounters a antigen that it can recognize, the cell is stimulated to multiply, and the population of lymphocytes bearing that particular receptor increases.

Autoimmune disorders like multiple sclerosis and rheumatoid arthritis occur when something goes wrong with the recognition system and the immune system attacks healthy tissues.