Circulating lymphocytes are the chief cellular constituents of lymphatic tissue.
To understand the function of lymphocytes, one must realize that most lymphocytes (approximately 70%) in blood or lymph represent a circulating pool of immunocompetent cells. These cells participate in a cycle during which they exit the systemic circulation to enter the lymphatic tissue. While there, they are responsible for immunologic surveillance of surrounding tissues. The cells then return to the systemic circulation. This population of cells is represented mainly by long-lived, mature lymphocytes (mainly T cells) that have developed the capacity to recognize and respond to foreign antigens and are in transit from one site of lymphatic tissue to another.
The remaining 30% of lymphocytes in the blood vessels do not circulate between the lymphatic tissues and the systemic circulation. This population comprises mainly short-lived, immature cells or activated cells destined for a specific tissue. These cells leave the capillaries and migrate directly to the tissues, especially into the connective tissue that underlies the lining epithelium of the respiratory, gastrointestinal, and urogenital tracts as well as into the intercellular spaces of these epithelia. Functionally, three major types of lymphocytes are present in the body: T lymphocytes, B lymphocytes, and NK cells. The functional classifcation of lymphocytes is independent of their morphologic (size) characteristics.
T lymphocytes differentiate in the thymus and account for the majority of circulating lymphocytes.
T lymphocytes (T cells) are named for the thymus, where they differentiate. They have a long lifespan and are involved in cell-mediated immunity. They account for 60% to 80% of circulating lymphocytes. T cells express CD2, CD3, CD5, and CD7 markers and T-cell receptors (TCRs); however, they are subclassified according to the presence or absence of two other important surface markers: CD4 and CD8.  Helper CD4 T lymphocytes are T cells that also express
CD4 markers. These cells are further subdivided by their ability to secrete cytokines (see page 454). Helper T cells that synthesize interleukin 2 (IL-2), interferon  (IFN-), and tumor necrosis factor  (TNF-) are called TH1 cells.
These cells interact with cytotoxic CD8 T lymphocytes (CTLs), NK cells, and macrophages in cell-mediated im mune responses and are essential for controlling intracellu lar pathogens such as viruses and certain microorganisms.
The other group of helper T cells synthesize IL-4, IL-5, IL 10, and IL-13 and are called TH2 cells. They interact with
B lymphocytes and are essential for initiating antibody-me pathogens.
 Cytotoxic CD8 T lymphocytes (CTLs) are T cells that also express CD8 markers. They kill other target cells such as virus-infected cells, cancer-transformed cells, cells infected with intracellular microorganisms, parasites, and transplanted cells.
 Regulatory (suppressor) T lymphocytes represent a phenotypically diverse population of T lymphocytes that can functionally suppress an  immune response to foreign and self-antigen by influencing the activity of other cells in the immune system. For example, T lymphocyte with CD4 CD25FOXP3 markers represent a classical example of the regulatory cells that can diminish the ability of T lymphocytes to initiate immune responses. The FOXP3 marker indicates an expression of forkhead family transcription factors that are characteristic of many T cells. Another tumor-associated T lymphocyte with CD8CD45RO markers is able to suppress T-cell activation. Other suppressor T cells may also function in suppressing B-cell differentiation and in regulating erythroid cell maturation in the bone marrow.