Type II Hypersensitivity (Cytotoxic)

Type II (cytotoxic) hypersensitivity is mediated by antibodies, typically IgG or IgM, directed against antigens on cell surfaces or in the extracellular matrix. The bound antibody injures tissue by activating complement, recruiting phagocytes and natural killer cells, or by altering the function of the target.

Definition

Type II hypersensitivity is antibody-mediated tissue injury in which IgG or IgM binds antigens on cell surfaces or extracellular matrix and causes damage through complement activation, opsonisation and phagocytosis, antibody-dependent cellular cytotoxicity, or alteration of cellular function.

Scope

This entry covers the antibody-mediated mechanisms of Type II hypersensitivity: complement-dependent lysis, opsonisation and phagocytosis, antibody-dependent cellular cytotoxicity, and antibody-mediated functional change. It situates these within immunopathology as a mechanistic reference and is not clinical guidance.

Core questions

• How do antibodies against cell-surface antigens destroy or alter the target cell?
• What roles do complement, phagocytes, and natural killer cells play?
• How can antibody binding change cellular function without destroying the cell?
• Why are blood cells and basement membranes common targets?

Key concepts

• IgG and IgM against surface or matrix antigens
• Complement-dependent cytotoxicity
• Opsonisation and phagocytosis
• Antibody-dependent cellular cytotoxicity (ADCC)
• Antibody-mediated functional change (stimulating or blocking)
• Direct and indirect antiglobulin (Coombs) test

Mechanisms

Antibodies bind antigens displayed on a cell surface or in the extracellular matrix and injure tissue by several routes. Complement fixation can lyse the target directly or coat it with fragments that promote phagocytosis; bound IgG opsonises the cell for Fc-receptor-bearing phagocytes; and natural killer cells recognise antibody-coated cells through Fc receptors to mediate antibody-dependent cellular cytotoxicity. In a functional variant, antibodies bind cell-surface receptors and either stimulate or block them without killing the cell, altering physiology. The antiglobulin (Coombs) test detects such cell-bound or circulating antibodies.

Clinical relevance

Type II mechanisms underlie disorders such as autoimmune haemolytic anaemia, immune thrombocytopenia, transfusion reactions and haemolytic disease of the newborn, Goodpasture disease (anti-basement-membrane antibody), and functional conditions where antibodies target receptors. The entry describes mechanisms for orientation and does not provide diagnostic or treatment guidance for these conditions.

Epidemiology

The frequency of Type II-mediated disorders varies widely by specific condition, from relatively common transfusion-related events to rare organ-specific antibody diseases; burden is addressed in the individual disease entries.

Evidence & guidelines

The mechanistic classification rests on immunology textbooks; disease-specific evidence and transfusion or transplantation guidance lie in the relevant clinical entries and external standards.

History

Antibody-mediated cell destruction was demonstrated through transfusion incompatibility and haemolytic disease of the newborn in the early-to-mid twentieth century, and the development of the antiglobulin (Coombs) test in 1945 gave a practical means to detect cell-bound antibody, anchoring this category in the later Gell and Coombs scheme.

Key figures

• Robin Coombs
• Philip Gell

Seminal works

• coombs-gell-1963

Frequently asked questions

How does Type II hypersensitivity differ from Type III?

In Type II, antibody binds antigen that is fixed on a cell surface or in tissue; in Type III, antibody binds soluble antigen in the circulation to form immune complexes that then deposit in tissues. The effector mechanisms overlap, but the location of the antigen differs.

Can antibodies cause disease without destroying cells?

Yes. In the functional form of Type II hypersensitivity, antibodies bind cell-surface receptors and either over-stimulate or block them, changing cellular function without lysing the cell.