Immunology is a vital branch of biology and medicine dedicated to studying the immune systems across all organisms, exploring their functions in both health and disease, as well as malfunctions like autoimmune disorders. The field was pioneered by Ilya Ilyich Mechnikov and Paul Ehrlich, who shared the Nobel Prize in 1908 for their groundbreaking work on immunity; Mechnikov notably discovered phagocytosis, the process where cells engulf foreign bodies, while Ehrlich demonstrated immunization against toxins. Early observations of immunity date back to 430 BCE, when Thucydides noted that survivors of the Plague of Athens could nurse the sick without falling ill again.

At its core, immunology investigates the body's ability to distinguish "self" from "non-self," involving a primitive innate immune system and a more specialized adaptive immune system in vertebrates. This recognition relies on lymphocytes identifying antigens (substances triggering an immune response) and secreting antibodies, which are proteins that neutralize pathogens or mark them for destruction. Key components, including lymphoid organs like the thymus and spleen, along with circulating immune cells, make immunology crucial for diverse medical disciplines such as organ transplantation, oncology, and virology.