Mechanism of Type III Hypersensitivity:
The factors responsible for the deposition of immune complexes in various tissues are not fully known. The immune complexes deposit in renal glomerulus, joints, and small blood vessels. The tissue deposited complexes activate the classical complement cascade (through the binding of CI to the Fc region of immunoglobulin in the complex).
The activation of the classical complement pathway leads to the following events:
Figs 17.2A and B: Removal of Circulating Immune Complexes (CICs) by Macrophages in Spleen and Liver.
Figs 17.2A and B: Removal of circulating immune complexes (CICs) by macrophages in spleen and liver.
(A) The binding of antibody with antigen (and formation of immune complexes) activates the classical complement pathway. The C3b fragments formed during complement activation falls on the surface of antigens. The RBC membrane has receptors for C3b. The C3b molecules on the surface of antigens bind to the C3b receptors on RBC membrane.
Thus the circulating immune complexes are bound to the RBC through C3b and C3b receptors, and (B) As the RBC moves through the sinusoids of the liver and spleen, the macrophage lining the sinusoids bind to the Fc regions of CICs through the Fc receptors on macrophage membrane (The complement receptors on macrophage membrane also bind to the complement components on CICs).