T lymphocytes can be divided into three main subtypes: effector, memory, and regulatory cells. Each type performs a distinct function during an immune response to foreign antigens. Effectors cells include helper T cells and Cytotoxic T cells.
These are differentiated by the expression of unique cell surface markers, such as CD4 for helper T cells and CD8 for cytotoxic T cells.
Helper T Cells (CD4+ T cells)
These cells help in maturation of B cells into plasma cells and memory B cells and to activate cytotoxic T cells and macrophages. They are activated when presented with peptide antigens by MHC Class II molecules, which are expressed on the surface of antigen presenting cells (APCs). Once activated, they divide rapidly and secrete small proteins (cytokines) that regulate or assist in active immune response. They are required for almost all adaptive immune responses. They not only help activate B cells to secrete antibodies and macrophages to destroy ingested microbes, but they also help activate cytotoxic T cells to kill infected target cells.
Two additional TH cell subsets have been recently identified.
- T helper type 17 cells (TH17), so named because they secrete IL-17, play an important role in cell-mediated immunity and may help the defense against fungi.
- T follicular helper cells (TFH) play an important role in humoral immunity and regulate B-cell development in germinal centers.
Cytotoxic T Cells (CD8+ T cells)
CD8+ cytotoxic cells cause lysis of virus-infected and tumor cells, impart immune defense against intracellular pathogens, including viruses and bacteria, tumor surveillance and are also involved in transplant rejection. They recognize their targets by binding to antigen associated with MHC Class I molecules which are present on the surface of all nucleated cells. They kill the cells that consist of foreign antigens, such as cells infected by viruses and other intracellular microbes.
Memory T Cells
Naive T cells on exposure to an antigen differentiate into effector cells (CD4+ and CD8+ cells) and memory T cells. Memory T cells are long-lived and can quickly expand to large numbers of effector T cells upon re-exposure to that same antigen. They aid in immune system with memory against previously encountered pathogens. Memory T cells may be either CD4+ or CD8+.
Regulatory T cells (TREG)
These cells has unique capacity to inhibit an immune response. They can arise during maturation in thymus from autoreactive cells (natural TREG), but also can be induced at the site of an immune response in an antigen-dependent manner. Induced TREG are identified by the presence of CD4 and CD25 on their surfaces, as well as the expression of internal transcription factor FoxP3.
They are critical to quell autoreactive responses that have not been avoided via other mechanisms. TREG cells may also play a role in limiting normal T-cell response to a pathogen.
Natural Killer T Cells
They bridge the adaptive immune system with innate immune system. While most T cells function based on recognition of MHC class molecules, natural killer T cells are able to recognize other antigen classes. On activation they are also able to perform the same functions as CD4+ and CD8+ cells. These cells are distinct from natural killer cells.