- Helper T cells, also known as Th cells or CD4+ T cells, are a subset of T lymphocytes that play a pivotal role in orchestrating and regulating the immune response.
- They do not directly kill infected or abnormal cells but instead coordinate and regulate the activities of other immune cells.
- Helper T cells are called “CD4+” because they express the CD4 protein on their surface, which helps them recognize antigens presented by specialized immune cells known as antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells. These antigens are displayed on molecules called MHC class II, which Helper T cells specifically recognize.
- When a Helper T cell encounters an antigen presented by an APC, it becomes activated and begins to secrete various cytokines. These signaling molecules coordinate the activities of other immune cells, enhancing their ability to respond to infections and other threats. The cytokines stimulate and enhance the function of various immune cells, helping activate cytotoxic T cells to destroy infected cells, promoting B cell antibody production, and recruiting macrophages to engulf and destroy pathogens.
- There are several subtypes of Helper T cells, each with distinct functions based on the cytokines they produce. The two main subtypes are Th1 and Th2 cells.
- Th1 cells primarily produce interferon-gamma (IFN-γ) and are essential for cell-mediated immunity, particularly in defending against intracellular pathogens like viruses and certain bacteria.
- Th2 cells secrete cytokines such as interleukin-4 (IL-4), IL-5, and IL-13, which are important for humoral immunity and helping B cells produce antibodies to combat extracellular pathogens like parasites.
- Other specialized subsets include Th17 and T follicular helper (Tfh) cells.
- Th17 cells, which produce interleukin-17 (IL-17), play a role in the immune response against fungi and certain bacteria, and are also involved in inflammatory and autoimmune conditions. Tfh cells are essential for the formation and function of germinal centers within lymphoid tissues, where they assist B cells in producing high-affinity antibodies through processes such as somatic hypermutation and class-switch recombination.
- Helper T cells are crucial for establishing immunological memory and maintaining immune homeostasis. After an immune response, some Helper T cells become memory cells, enabling the immune system to mount a faster and stronger defense against recurring pathogens.
- Regulatory T cells (Treg cells), a subset of Helper T cells, secrete inhibitory cytokines like interleukin-10 (IL-10) and transforming growth factor-beta (TGF-β) to suppress immune responses and maintain tolerance to self-antigens, helping prevent autoimmune diseases.
- Dysfunction of Helper T cells can lead to severe immune deficiencies, as seen in HIV infection, which specifically targets and destroys CD4+ T cells.
- Overall, Helper T cells act as conductors of the immune orchestra, directing the activities of other immune cells and ensuring that the body can mount a precise and regulated defense against various pathogens.
