Immunology is a branch of biology[1] that covers the study of immune systems[2] in all organisms.[3] Immunology charts, measures, and contextualizes the physiological functioning of the immune system in states of both health and diseases; malfunctions of the immune system in immunological disorders (such as autoimmune diseases,[4] hypersensitivities,[5] immune deficiency,[6] and transplant rejection[7]); and the physical, chemical, and physiological characteristics of the components of the immune system in vitro,[8] in situ, and in vivo.[9] Immunology has applications in numerous disciplines of medicine, particularly in the fields of organ transplantation, oncology, rheumatology, virology, bacteriology, parasitology, psychiatry, and dermatology.

The term was coined by Russian biologist Ilya Ilyich Mechnikov,[10] who advanced studies on immunology and received the Nobel Prize for his work in 1908. He pinned small thorns into starfish larvae and noticed unusual cells surrounding the thorns. This was the active response of the body trying to maintain its integrity. It was Mechnikov who first observed the phenomenon of phagocytosis,[11] in which the body defends itself against a foreign body.

Prior to the designation of immunity,[12] from the etymological root immunis, which is Latin for “exempt”, early physicians characterized organs that would later be proven as essential components of the immune system. The important lymphoid organs of the immune system are the thymus,[13] bone marrow, and chief lymphatic tissues such as spleentonsilslymph vesselslymph nodesadenoids, and liver. When health conditions worsen to emergency status, portions of immune system organs, including the thymus, spleen, bone marrow, lymph nodes, and other lymphatic tissues, can be surgically excised for examination while patients are still alive.

Many components of the immune system are typically cellular in nature and not associated with any specific organ, but rather are embedded or circulating in various tissues located throughout the body.