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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.