CD100 also known as Sema4D is a transmembrane protein belonging to group IV of the semaphorin family. CD100 plays a crucial role in the immune system, being the only member of the semaphorin family expressed in immune tissues. CD100 is vital for T cell proliferation and B cell aggregation, as CD100 regulates B cell responses by counteracting inhibitory signals from CD72, which typically suppresses B cell activity. When CD100 binds to CD72, CD100 induces tyrosine dephosphorylation of CD72, leading to the dissociation of SHP-1, a negative regulator of B cell signaling. CD100 expression is observed in various tissues, including the kidney, heart, lung, and neural tissues, highlighting CD100′s importance in both embryonic and adult development.