CD5L is a member of the scavenger receptor cysteine-rich domain superfamily (SRCR-SF) initially identified as an inducible cell surface ligand of CD5. It was shown that CD5L functions in the thymus as the inducer of resistance to apoptosis within CD4+/CD8+ thymocytes and as the supporter of the viability of these cells before thymic selection. CD5L was also shown to support macrophage survival and enhance their phagocytic function. More recent experiments using recombinant CD5L significantly inhibited apoptosis of NKT and T cells obtained from C. parvum-stimulated livers in vitro, suggesting that CD5L functions to induce resistance to apoptosis in these cells and supports host defense against inflammation during infection.