humoral

Humoral refers to aspects of physiology and immunology mediated by body fluids, particularly those in blood and lymph. The term originates from the ancient concept of humors, four bodily fluids (blood, phlegm, yellow bile, black bile) whose balance was thought to influence health. Although humoral theory as a pathogenic framework is obsolete, the term persists in modern science to describe immune mechanisms operating in extracellular spaces and plasma.

In modern immunology, humoral immunity is the part of the adaptive immune response that involves secreted antibodies produced by B lymphocytes. When B cells encounter an antigen, they differentiate into plasma cells that release immunoglobulins into the circulation. Antibodies can neutralize pathogens, tag them for attack by other immune cells, and activate the complement system. This form of immunity is particularly effective against extracellular bacteria, toxins, and viruses before they infect host cells. Humoral memory is maintained by memory B cells, enabling a faster and stronger response upon re-exposure to the same antigen.

Humoral components also include a range of soluble factors in plasma and lymph, such as immunoglobulins, complement proteins, and other mediators involved in signaling and defense. In immunology, humoral responses are often contrasted with cellular (cell-mediated) immunity, which primarily involves T lymphocytes and other immune cells that act directly on infected targets. The study of humoral immunity covers serology, vaccine-induced antibody production, antibody diversity, affinity maturation, and regulation of antibody isotypes.