Semaphorin 4A (SEMA4A) is a class IV semaphorin of soluble and transmembrane proteins originally identified as SemB. It is a single-pass type I membrane protein containing an immunoglobulin-like C2-type domain, a PSI domain, and a Sema domain. SEMA4A is expressed on dendritic cells, activated B cells, and T cells. It is also found on different tissues, such as those in brain, lung, kindney, spleen, and testis. In the immune system, SEMA4A functions as an activator in the T-cell-mediated immune response and suppresses vascular endothelial growth factor (VEGF)-mediated endothelial cell migration and proliferation in vitro and angiogenesis in vivo. Like other semaphorin family members it is also involved in guidance of axonal migration during neuronal development and in immune responses.