CD45 is a 180-240kD glycoprotein member of the protein tyrosine phosphatase (PTP) family known for its involvement in regulating a variety of cellular processes including cell growth, differentiation, mitotic cycle, and oncogenic transformation. CD45 and its isoforms are vital regulators of T- and B-cell antigen receptor signaling. CD45 functions through its extracellular domain or through its cytoplasmic domain, and serves as a negative regulator of cytokine receptor signaling via JAK kinase supression. The large extracellular domain is highly glycosylated, and its multiple isoforms allow extensive variation in the structure of its side chains. CD45 isoforms show cell-type and differentiation-stage specific expression that can be used as markers that identify and distinguish between different types of immune cells. CD45R is an isoform of CD45 with a molecular weight of 220 kD. CD45R contains all three possible exons (A, B, and C); making it the longest protein generated from alternative splicing with a migration at 200 kD when isolated from T cells. Furthermore, B cells express CD45R with heavier glycosylation, bringing the molecular weight to 220 kD, hence the name B220. Notably, B220 expression is not only restricted to B cells and may also be expressed on activated T cells, on a subset of dendritic cells, and on other antigen-presenting cells. Additionally, activated and memory T lymphocytes express CD45RO which facilitates T cell activation. CD45RO lacks all three possible exons (A, B, and C), making it the shortest CD45 isoform.
