CD100, also known as Sema4D, is a transmembrane protein belonging to the Class IV subfamily of Semaphorins. CD100 is active as a homodimer, however cleavage by certain metalloproteinases produce a soluble dimer that is also active. It is expressed on T and B cells, as well as macrophages, and with upregulated expression on activated T cells. CD100 plays important roles in immune and inflammatory responses, being involved in B cell and dendritic cell activation. It also induces B cells to aggregate and may improve their viability. CD72 is one of 2 known receptors for CD100 and binding of the ligand to this receptor is reported to be an important factor in maintaining immunological homeostasis.