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.