Lymphocyte population is an important type of white blood cell in the immune system. According to specific cell surface protein, such as cluster of differentiation (CD) markers, lymphocyte population can also be devided into three main subgroup: natrual killing cells (CD3-/CD16+56+), B cells (CD19+) and T cells (CD3+). And T cells can be sub-devided into helper T cells (CD4+) and cytotoxic T cells (CD8+). They have different functions in immune reaction. Helper T cells can release cytokines and growth factors that regulate other immune cells, while cytotoxic T cells can have the function of lysis of virally infected cells, tumour cells and allografts. T cells can also be devided to other sub-population, such as Th1/Th2, ct1, ts1/ts2/ts3, and so on.

The components of lymphocyte have much relation to many diseases, such as autoimmune diseases, immunodeficiency disorders, allergic disease, aplastic anemia, malignant tumor, and so on. An increase in lymphocyte concentration is usually a sign of a viral infection, and a low normal to low absolute lymphocyte concentration is associated with increased rates of infection after surgery or trauma.

Flow cytometry can be used in immunophenotying of lymphocyte poppulation. Varies of monoclonal antibodies and 2 to 3 color staining approaches have made the detection convenient, which play an important role in disease diagnosis and therapy.