CD40-Ligand, also known as CD40L, CD154, TRAP, or TNFSF5 is a member of the TNF superfamily transiently expressed on the surface of activated CD4
+ T lymphocytes. It is either expressed membrane-bound or proteolytically released as a soluble form which comprises 2/3 of the extracellular domain. Its receptor CD40 is found on antigen-presenting cells such as B cells, dendritic cells, and macrophages. CD40L-CD40 interaction is important in T cell-APC (antigen-presenting cell) interaction and is e.g. involved in B cell differentiation and proliferation, isotype class-switching, and protection of B cells from apoptosis.