CD45

In immunology, the CD45 antigen (CD stands for cluster of differentiation) is a protein which was originally called leukocyte common antigen.

It is a type I transmembrane protein which is in various forms present on all differentiated hematopoietic cells except erythrocytes and plasma cells that assists in the activation of those cells (a form of co-stimulation).

It is expressed in lymphomas, B-cell chronic lymphocytic leukemia, hairy cell leukemia, and acute nonlymphocytic leukemia.

The CD45 family consists of multiple members that are all products of a single complex gene. This gene contains 34 exons and the primary transcripts are alternatively spliced to generate up to eight different mature mRNAs and after translation eight different protein products.

Isoforms of CD45 exist:

• on B lymphocytes the isoform B220 is expressed on most mouse B cells and a subset of human B cells
• on naive T lymphocytes, where it is called CD45RA
• on activated and memory T lymphocytes, where it is called CD45RO

The cytoplasmic domain of CD45 is one of the largest known and it has an intrinsic phosphatase activity that removes an inhibitory phosphate group on a tyrosine kinase called Lck (in T cells) or Syk (in B cells) and activates it.

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