Flow cytometry is a technique that rapidly performs analysis of single cells that are suspended in a solution by using lasers. It identifies markers present on the surface of the cells as well as inside the cells. Commonly this technique is used to type the cancers arising from blood & bone marrow called Leukemias and Lymphomas. By this method leukemias / lymphomas can be typed as per their origin from different types of white blood cells namely B-lymphocytes, T-lymphocytes, NK cells & other cells like histiocytes. CD3 is a marker of cancers arising from T-lymphocytes. Flow cytometry is a technique that rapidly performs analysis of single cells that are suspended in a solution by using lasers. It identifies markers present on the surface of the cells as well as inside the cells. Commonly this technique is used to type the cancers arising from blood & bone marrow called Leukemias and Lymphomas. By this method leukemias / lymphomas can be typed as per their origin from different types of white blood cells namely B-lymphocytes, T-lymphocytes, NK cells & other cells like histiocytes. CD4 positive T-lymphocytes are called “Helper cells” which are routinely used to monitor HIV/AIDS patients. This test is also used to monitor patients taking immunosuppressive treatment following organ transplant and for T-cell malignancies.