CD5, also known as T-cell surface glycoprotein CD5 or Leu-1, is a type I transmembrane glycoprotein expressed on the surface of T cells, thymocytes, and a subset of B cells called B-1a cells. It belongs to the scavenger receptor superfamily and has a molecular mass of approximately 67 kDa. CD5 plays a crucial role in modulating immune responses by regulating the activation of T cells and B cells. In T cells, CD5 expression is upregulated upon activation and helps to fine-tune T cell receptor (TCR) signaling, promoting cell survival and non-responsiveness. In B cells, particularly B-1a cells, CD5 helps to prevent overactivation by dampening signals from the B cell receptor (BCR), ensuring that these cells only respond to strong stimuli. The exact ligand for CD5 remains unclear, but it is suggested that it may interact homophilically or with CD72 on B cells.