When antibodies bind antigen that is fixed in a tissue — on a cell surface or in the extracellular matrix — the resulting damage is concentrated at that site. Two consequences follow almost immediately. First, the Fc portion of bound IgG or IgM is recognized by C1q, initiating the classical complement pathway. Second, the same Fc portion is engaged by Fc receptors on neutrophils, macrophages, and mast cells, which then release inflammatory mediators and enzymes.
Complement converts a small number of bound antibodies into a much larger effector response. C3b deposited on the target surface acts as an opsonin, marking the tissue for phagocytosis. C5a is a potent chemoattractant that draws neutrophils into the site and activates them. The terminal components C5b through C9 assemble into the membrane attack complex, which forms pores in cell membranes and can lyse the antibody-coated cell directly.
The result is a self-amplifying loop: antibody recruits complement, complement attracts and activates phagocytes, and those phagocytes release enzymes and reactive oxygen species that damage surrounding tissue. Because the antigen is fixed, the injury stays localized to the tissue that carries it. This pattern is the basis of what is classified as type II hypersensitivity.