The class 4 semaphorins are integral membrane proteins that are widely expressed throughout the nervous system. SEMA4B is a single-pass type I membrane protein and contains one Ig-like C2-type (immunoglobulin-like) domain, one PSI domain and one Sema domain. Human SEMA4B is expressed in neurons. SEMA4B inhibits axonal extension by providing local signals to specify territories inaccessible for growing axons. SEMA4B negatively regulates basophil-mediated Th2 and humoral memory responses.