Human B cells express a CD45 isoform that is similar to murine B220 and is downregulated with acquisition of the memory B‐cell marker CD27

The B220 isoform in humans is developmentally regulated in humans, tied to the acquisition of a memory phenotype, and as such can be used as a differentiation-specific CD45 isoform, akin to the use of CD45 isoforms to distinguish between naive and memory T-cell subsets. Patients with immunodeficiency disorders, associated with defective memory B-cell generation and absent or reduced CD27(+) B cells, showed a corresponding lack of B220 downregulation consistent with altered differentiation of B-cell subsets.