Veterinary Immunology and Serology

T CELL SUBSETS

• Several different subsets of T cells have been described, each with a distinct function.
• Helper T cells are the “middlemen” of the adaptive immune system which are CD4⁺ T cells. The activated cells divide rapidly and secrete cytokines, which regulate or “help” in immune response. There are different subsets of Th cells:
  • Th1: Th1 cells secrete cytokines which help in the induction of the cell mediated immune response.
  • Th2: Th2 cells secrete cytokines which help in the production of humoral immune response.
  • Th17: This subset of T helper cells are associated with inflammation in several autoimmune and inflammatory diseases.
• Cytotoxic T cells (T_c cells) destroy virally infected cells and tumor cells, and are also implicated in transplant rejection. These cells are CD8⁺.
•  γ/δ T cells  constitute 5%-15%  blood lymphocytes in human and mice but about 60% in young ruminants in the blood circulation. Many γ/δ T cells have nonpolymorphic TCRs that recognize microbial glycolipids presented by CD1 positive antigen-presenting cells and release cytokines and lyse target cells.
• Memory T cells are a subset of antigen-specific T cells that persist long-term after an infection has resolved. They quickly expand to large numbers of effector T cells on subsequent exposure to the same antigen. Memory cells may either be CD4⁺ or CD8⁺. 
• Regulatory T cells play a master role in regulating the immune system and maintaining the balance between peripheral tolerance and immunity.
• Natural Killer T cells (NK T cells) are a special kind of lymphocyte that bridges the adaptive immune system with the innate immune system. Unlike conventional T cells, NK T cells recognize glycolipid antigens presented by molecule called CD1s. Once activated, these cells can perform functions of both T_H and T_C cells (i.e. cytokine production and release of cytolytic/cell killing molecules).