T helper cell
It is unknown what role the relatively bulky extracellular region of CD45 plays during cell interactions, but CD45 has various isoforms that change in size depending on the Th cell's activation and maturation status. For example, CD45 shortens in length following Th activation (CD45RA+ to CD45RO+), but whether this change in length influences activation is unknown. It has been proposed that the larger CD45RA may decrease the accessibility of the T cell receptor for the antigen-MHC molecule, thereby necessitating an increase in the affinity (and specificity) of the T cell for activation. However, once the activation has occurred, CD45 shortens, allowing easier interactions and activation as an effector T helper cell.