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CD5 is a cluster of differentiation expressed on the surface of T cells （various species） and in a subset of murine B cells known as B-1a. CD5 serves to mitigate activating signals from the BCR so that the B-1 cells can only be activated by very strong stimuli （such as bacterial proteins） and not by normal tissue proteins. CD5 was used as a T-cell marker until monoclonal antibodies against CD3 were developed. There is no confirmed ligand for CD5 but there is evidence that CD72, a C-type lectin, may be a ligand or that CD5 may be homophilic, binding CD5 on the surface of other cells.