CD24, also known as heat stable antigen, is a phosphatidylinositol-linked cell surface glycoprotein that is expressed at all stages of B cell development and on most thymocytes. The absence of expression from mature T cells is closely associated with their maturation from CD4+CD8+CD24+ thymocytes to either CD4+CD8-CD24- or CD4-CD8+CD24- T cells. CD24 is also present on mouse granulocytes, monocytes, Langerhans cells and erythrocytes. Its ligand is P-selectin (CD62P). CD24 may play a role in regulation of B cell proliferation and differentiation, and it has been proposed that heat stable antigen promotes homotypic adhesion between B lymphocytes.