Type IIB Hypersensitivity: Myasthenia Gravis
Myasthenia gravis is an autoimmune disease caused by inhibitory (antagonis-tic) autoantibodies that bind and block the acetylcholine receptor (AChR), causing muscular weakness and fatigue. The AChR is found at postsynaptic membranes of neuromuscular junctions and binds acetylcholine released from a nerve ending, transiently opening a calcium channel. The signal is terminated by acetylcholine esterase, an enzyme located in the basal lamina between the nerve ending and the postsynaptic membrane. As in mothers with Gravesâ€™ disease, transplacental pas-sage of IgG autoantibodies from mothers with myasthenia gravis can cause transient neonatal myasthenia gravis. Anti-AChR autoantibodies cause disease by down-regulating expression of the receptor and by complement-mediated lysis of the cells bearing the AChR. Intermolecular cross-linking of AChR by the autoantibodies may lead to antigenic modulation.