The immune system is clearly “a good thing,” but like mercenary armies, it can turn to bite the hand that feeds it, and cause damage to the host (Figure 2.18).
Thus when there is an especially heightened response or persistent exposure to exogenous antigens, tissue damaging or hypersensitivity reactions may result. Examples are allergy to grass pollens, immune complex glomerulonephritis occurring after streptococcal infection, and chronic granulomas produced during tuberculosis or schistosomiasis.
In other cases, responses to autoantigens may arise through a breakdown in the mechanisms that control self tolerance, and a wide variety of autoimmune diseases, such as type 1 (insulin‐dependent) diabetes and multiple sclerosis and many of the rheumatologic disorders, may result from an autoimmune attack.
Another immunopathologic reaction of some consequence is graft rejection, in which the MHC antigens on the donor transplant may well provoke a fierce reaction.