Protective immune responses mediated by antibodies produced by B cells and by complement proteins circulating in body fluids.
Explanation
Humoral immunity is one arm of the adaptive immune system that targets extracellular pathogens and toxins. It is orchestrated by B lymphocytes, which recognise specific antigens via membrane-bound immunoglobulin receptors. Upon stimulation, often with help from CD4+ T helper cells, B cells undergo clonal expansion and differentiate into plasma cells that secrete large quantities of soluble antibodies. Antibodies bind antigens with high specificity, neutralising viruses and toxins by preventing attachment to host cells, and agglutinating microbes to facilitate clearance. They also opsonise bacteria and fungi, marking them for phagocytosis by neutrophils and macrophages, and they activate the complement cascade through the classical pathway. The complement system is a group of plasma proteins that, once activated, generate inflammatory mediators, promote opsonisation, recruit immune cells and form membrane attack complexes that lyse susceptible microbes. Antibody classes include IgM, the first isotype produced in a primary response; IgG, which dominates secondary responses and crosses the placenta; IgA, abundant in mucosal secretions; IgE, associated with allergy and defence against helminths; and IgD, whose function is less well defined. Memory B cells formed during the primary response persist for years, enabling rapid and robust antibody production upon re-exposure to the same antigen. Humoral responses can be elicited by T-dependent protein antigens, which induce class switching and affinity maturation, or by T-independent polysaccharide antigens, which primarily stimulate IgM production. Together with cell-mediated immunity, humoral immunity provides comprehensive protection against infection.
Notable Aspects and Examples
Vaccination works by inducing humoral responses; for example, hepatitis B and measles vaccines generate neutralising antibodies that prevent infection. Passive immunisation using pooled immunoglobulin preparations can provide immediate protection to individuals with primary immunodeficiency. Naturally occurring antibodies against ABO blood group antigens are responsible for transfusion reactions if incompatible blood types are mixed. Autoimmune diseases such as myasthenia gravis and systemic lupus erythematosus arise when antibodies mistakenly target self proteins. Complement deficiencies, such as C3 deficiency, impair the ability to clear encapsulated bacteria and predispose to recurrent infections.
Humoral immunity reflects the capacity of the adaptive immune system to recognise specific antigens and produce targeted soluble mediators. It complements cellular responses and underlies the success of many vaccines and therapeutic antibodies.
Related Terms: Antibody, B cell, Plasma cell, Complement, Adaptive immunity