CD45R, also known as B220, is a protein tyrosine phosphatase that is predominantly found on B cells, at varying levels from pro-B cells to activated B cells, and may also be detected on certain T cell and NK cell subsets. CD45R plays a crucial role in B cell receptor and T cell receptor signaling and is essential for lymphocyte development and function. It helps regulate the threshold of sensitivity to external stimuli, affecting cellular responses. In mice, CD45R/B220 is a common marker for B cells, whereas in humans, B cells do not express B220 in the same way; it appears to be differentiation-specific and is therefore expressed on only some B cell subsets. CD45R is also used in the classification of leukemias and lymphomas and in research on functional subsets of B and T cells.