Red cells from patients with paroxystic nocturnal hemoglobinuria get lysed by complement because they lack protecting proteins, i.e. complement receptors, which inhibit complement as soon as it is assembled on the red cell surface – during nighttime, when pH in kidneys slightly drops, they become a victim of complement.
CR1 (see transport tag) forms part of these protecting proteins. Thus, immune complexes interact with two larger groups of receptors: the receptors specific for Immunoglobulin Fc-fragments and those for complement. Binding of immune complexes to these cells essentially are the following: (i) transport from one location to the other (see transport tag), (ii) endocytosis/phagocytosis and (iii) induction of specific effector functions.
The figure displays schematically the most important receptors: like the handshake between antigen and antibody, the interaction of ligand with receptor is not subject to covalent chemical bonding but depends on affinity, the strength of which one can measure using Scatchard plots.
Ligand can escape from receptor binding –> immune complex can be released from red blood cells. The principle of control over excessive activation by downregulating mechanisms in nature is again obvious from the figure which distinguishes cell activating (ITAM) from inhibiting (ITIM) receptors in the neighborhood of cell surface bound monomeric IgG (sIgG).
Three major consequences are possible from immune complex-cell interaction: reduced proliferation, enhanced maturation/proliferation and apoptosis induction.