CD24, also known as Heat-Stable Antigen and Nectadrin, is a heavily and variably glycosylated 30 kDa-60 kDa GPI-linked sialoprotein. Human CD24 is expressed on B lineage cells and granulocytes, on epithelial, neuronal, and muscle cells, and on a range of tumor cells. In mouse, CD24 is even more widely expressed, particularly on T cells, monocytes, and dendritic cells. CD24 expression is regulated during lineage development and with the activation of various cell types. Antibody crosslinking of CD24 enhances the induction of apoptosis in B and T lymphocytes which contributes to negative selection and the induction of immune tolerance. CD24 on antigen presenting cells cooperates with B7 molecules in the costimulation of T cells. CD24 associates in cis with Siglec-10 (or Siglec-G in mouse) and with the danger-associated molecules HMGB1, HSP70, or HSP90 which are released from necrotic or damaged cells. Formation of these ternary complexes fills a protective role: the resulting Siglec-10 signaling inhibits inflammatory responses that are otherwise induced by extracellular DAMPs. Mature human CD24 shares 30% and 42% amino acid sequence identity with mouse and rat CD24, respectively.