CD40 ligand (CD40L), also known as CD154,
gp39 and TRAP, is a 33 kDa type II membrane glycoprotein expressed
mainly on the cell surface of activated T lymphocytes but also
exists as a soluble form extracellularly. CD40L is the ligand
for CD40, a member of the TNF superfamily, which is expressed
on the cell surface of B cells, macrophages/monocytes, dendritic
cells, vascular endothelial cells, and epithelial cells. CD40L
plays an important role in B cell proliferation, antibody class
switching, modulation of apoptosis in the germinal center through
interaction with B cells expressing CD40, and activation of CD4+
T cells. Mutation within the CD40L gene is linked to hyper IgM
syndrome, an Xlinked immunodeficiency disease that is characterized
by elevated level of serum IgM and decreased level of other isotypes.
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