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 Graves’ 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.
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