Anti-CD45, also known as anti-leukocyte common antigen, is a commonly employed tool for distinguishing between various types of poorly defined tumors when morphological features or clinical findings suggest the possibility of malignant lymphoma. As a remarkably specific antibody, a positive result strongly points towards a hematolymphoid origin for the tumor. Nevertheless, the absence of CD45 does not definitively exclude a hematolymphoid tumor, as some hematolymphoid neoplasms, such as Hodgkin lymphoma, certain T-cell lymphomas, and specific leukemias, may not express it. This particular antibody is almost exclusively found on cells originating from the hematopoietic lineage and can be detected in the majority of both benign and malignant lymphocytes, including plasma cell precursors.