CD4 lymphocytes (CD4 T) belong to the class of white blood cells referred to as T-lymphocytes (T). Frequently referred to as helper T cells, CD4 lymphocytes discharge cytokines that are important for prompting or increasing responses of the immune system. These are also the cells through which the human immunodeficiency virus enters the body, leaving it vulnerable to infections such as pneumocystis pneumonia, cytomegalovirus, and Kaposi's sarcoma.
There are three types of lymphocytes: natural killer cells (NK), B-lymphocytes (B), and T-lymphocytes (T). NK cells destroy cells infected by viruses and cancerous cells. Necessary for humoral immunity, B cells are manufactured within the bone marrow, differentiating as plasma cells responsible for antibody production. Produced by the thymus gland, T cells are activated in cell-mediated immune responses in which they work to destroy pathogens, cells recognized as foreign invaders as well as those that have been abnormally altered, such as cancer.
Further divided into two subgroups, CD4 lymphocytes encompass T helper 1 and T helper 2 classifications. Even though both types excrete cytokines, they produce different kinds and both vary in their functions. While T helper 1 cells assist in cell-mediated immunity, T helper 2 cells stimulate B cell division, are active in the production of antibodies, and function in antibody-mediated immunity.
Several types of cytokines that are important to responses of the immune system are produced by CD4 lymphocytes. Certain types of cytokines have an effect upon the development of B cells; some affect T cell development and other types of cytokines influence macrophage functioning. Activating the development of NK cells, B cells, and T cells, interleukin two (IL-2) is a cytokine produced by T helper 1 cells. Another cytokine referred to as interleukin four (IL-4) is manufactured by T helper 2 cells, along with B cells and particle-ingesting macrophages. IL-2 is active in cell-mediated immunity, while IL-4 increases humoral immune function.
Human immunodeficiency virus (HIV), the microorganism responsible for causing acquired immunodeficiency syndrome (AIDS), obtains access to the immune system via CD4 lymphocytes, resulting in express suppression of immune system responses. As the invading virus initiates attachment to the surface of a CD4 lymphocyte, it infects the cell and manipulates the its genetic information for continued infection and viral replication. HIV then systematically destroys the CD4 lymphocytes of an infected individual. Once the CD4 count succumbs below 200, AIDS, the advanced form of HIV infection, is typically diagnosed.