How Do the Immune Cells of the Body Know What to Attack and What Not To?
All immune and blood cells develop from multipotent hematopoietic stem cells that originate in the bone marrow. Upon their departure from the bone marrow, immature T cells undergo a final maturation process in the thymus, a small organ located in the upper chest, before being dispersed to the body with the rest of the immune cells (e.g., B cells). Within the thymus, T cells undergo an important process that “educates” them to distinguish between self (the proteins of their own body) and nonself (the invading organism’s) antigens. Here, the T cells are selected for their ability to bind to the particular MHC proteins expressed by the individual. The particular array of MHCs varies slightly between individuals, and this variation is the basis of the immune response when a transplanted organ is rejected. MHCs and other less easily characterized molecules called minor histocompatibility antigens are genetically determined and this is the reason why donor organs from relatives of the recipi
