The function of the immune system is the detection and destruction of invading microorganisms and abnormal cells. Because of the great diversity of microbial invaders, the immune system is a complex mixture of protective mechanisms. These may be simply classified as innate immunity and adaptive immunity. Protection within the first few hours and days of microbial invasion is the responsibility of the “hard wired” innate immune system. Longer-term protection is the responsibility of the adaptive immune system.
In general, immune system disease takes three forms: insufficient immune function causing immunodeficiencies, manifested as increased susceptibility to infections; diseases resulting from excessive immune function, resulting in inappropriate cell and tissue damage or hypersensitivities; and diseases in which the immune system directly attacks normal body cells and organs, called autoimmune diseases. Some inflammation and limited tissue destruction are features of the normal innate and adaptive immune responses. Clinical disease occurs when this inflammation is excessive or in the wrong place or is directed against the wrong target.