CD100 commonly known as SEMA4D is a cell surface protein of resting T cells involved in lymphocyte activation. An 862aa signaling growth factor CD100 belongs to neuro-semaphorin family of proteins with an Ig-like C2-type (immunoglobulin-like) domain, a PSI domain and a Sema domain. Activation of plexin-B1 by CD100 induces different biological responses through the differential association of its receptor with ErbB-2 and Met. CD100 regulates certain immune responses by mediating in vitro and in vivo allo-geneic responses by modulating T cell-APC interactions and induces B cells aggregation, differentiation and improves their viability in vitro. Expression of CD100 on the surface of activated NK cells increases their cytotoxicity and mediates the killing of target cells by interacting with CD72. It is an important functional protein and a novel leukocyte semaphorin, of both the immune system, and the nervous system and is widely expressed in most of the tissues both hematopoietic and nonhematopoietic.