CD45RA protein is a specific isoform of the CD45 receptor, also known as protein tyrosine phosphatase receptor type C (PTPRC), which is generated through alternative splicing of the CD45 gene. It is a marker for naive T cells and central memory T cells (TCM), characterized by the presence of exon 4 (A) but the absence of exons 5 (B) and 6 (C) in its mRNA. CD45RA is expressed on the surface of nucleated hematopoietic cells, including B cells, NK cells, naive/resting T cells, medullary thymocytes, and plasmacytoid dendritic cells. It plays a crucial role in the immune system by modulating T cell receptor (TCR) and B cell receptor (BCR) signaling pathways, facilitating the activation and proliferation of immune cells. In adults, CD45RA+ cells may help prevent autoimmune diseases, while reduced numbers of CD45RA+ CD4+ T cells have been associated with conditions such as biliary atresia, primary biliary cirrhosis, fetal distress, and preeclampsia. Pathologists use CD45RA to differentiate naive T cells (CD45RA+) from memory T cells (CD45RO+), although this distinction may not be applicable in adult allogeneic hematopoietic cell transplant recipients.