The CD3 Flow Cytometry test is used to identify and quantify T-lymphocytes in a blood, bone marrow, or tissue sample. CD3 is a pan T-cell marker present on all mature T-cells and is a critical component of the T-cell receptor (TCR) complex. This test helps evaluate the immune status of an individual, detect T-cell abnormalities, and monitor diseases such as immunodeficiencies, leukemias, and lymphomas.