CD45RA epitopes are present on all the CD45 restricted molecules in which the exon A is expressed (ABC and AB containing isoforms). Most B lymphocytes express CD45RA isoforms. The differential expression of CD45 isoforms is associated with different functional properties of T lymphocytes. Peripheral T lymphocytes can be divided into two largely distinct populations of CD45RA- CD45RO+ and CD45RA+ CD45RO - cells. CD45RA is expressed by medullar thymocytes and naïve/resting peripheral CD4+ T lymphocytes.