CD100, also known as Sema4D or collapsin-4, is a 150 kDa transmembrane glycoprotein belonging to group IV of the semaphorin family. The CD100 dimer activates T-cells and modifies B-cell signaling through CD40 by promoting B-cell aggregation and survival and by down-regulating CD23 expression. It also regulates B cell responses by down-regulating the inhibitory signal from its receptor, CD72. This signal is turned off when CD100 binds to CD72 and induces tyrosine dephosphorylation of CD72. Soluble CD100, released by activated lymphocytes, significantly enhances B-cell responses induced by CD40. CD100 also binds plexin-1, a receptor that is abundantly expressed in fetal brain and kidney.Synonyms: A8, BB18, GR3, SEMAJ, Semaphorin-4D