Cytokine that acts as a ligand to CD40/TNFRSF5 (By similarity). Costimulates T-cell proliferation and cytokine production (By similarity). Induces the activation of NF-kappa-B (By similarity). Mediates B-cell proliferation in the absence of co-stimulus as well as IgE production in the presence of IL4 (By similarity). Involved in immunoglobulin class switching (By similarity). Acts as a ligand for integrins, specifically ITGA5:ITGB1 and ITGAV:ITGB3; both integrins and the CD40 receptor are required for activation of CD40-CD40LG signaling, which have cell-type dependent effects, such as B-cell activation, NF-kappa-B signaling and anti-apoptotic signaling. Homotrimer (By similarity). Homotrimer (By similarity). Interacts with CD28 (By similarity). CD40 ligand, soluble form: Exists as either a monomer or a homotrimer (By similarity). Forms a ternary complex between CD40 and integrins for CD40-CD40LG signaling (By similarity). Release of soluble CD40L from platelets is partially regulated by GP IIb/IIIa, actin polymerization, and a matrix metalloproteinases (MMP) inhibitor-sensitive pathway. The soluble form derives from the membrane form by proteolytic processing. Belongs to the tumor necrosis factor family.