Immune system is a complex network of organs containing cells that recognize foreign substances in the body and destroy them. Innate immunity refers to defense mechanisms that are present even before infection. Adaptive immunity develops later after exposure to microbes and is more powerful in combating infections. The major components of innate immunity are phagocytic cell, natural killer cells and several plasma proteins. The adaptive immune system consists of lymphocytes and their products including antibodies. Cellular immunity is mediated by T-lymphocytes and humoral immunity is mediated by B-lymphocytes. Major histocompatibility complex molecules play key role in regulating T-cell mediated immune responses. Immunodeficiency, autoimmunity and hypersensitivity are the disorders of human immunity.