Helper T cells are central coordinators of the body’s immune system. These specialized white blood cells, also known as CD4+ T cells, distinguish between the body’s own healthy cells and foreign invaders like bacteria, viruses, or other pathogens. They do not directly attack threats but instead orchestrate the responses of other immune cells, ensuring a targeted and effective defense. Their activity is integral to the adaptive immune response, which provides long-lasting protection against specific pathogens.
Activating Helper T Cells
Helper T cells must first become activated to perform their functions. Activation begins with specialized antigen-presenting cells (APCs), such as dendritic cells or macrophages, which patrol the body for signs of infection. When an APC encounters a pathogen, it engulfs and breaks it down into smaller fragments called antigens. These antigens are then displayed on the APC’s surface, specifically within a molecule called Major Histocompatibility Complex (MHC) class II.
Activation involves the helper T cell’s T cell receptor (TCR) recognizing and binding to this antigen-MHC class II complex on the APC. This initial binding provides the first signal for activation. However, a second signal, known as co-stimulation, is also required for robust activation and to prevent unintended immune responses. This co-stimulation often involves the interaction between a protein called CD28 on the helper T cell and B7 proteins on the APC. This two-signal system ensures that helper T cells are only activated in the presence of a genuine threat, preventing autoimmune reactions where the immune system mistakenly attacks healthy tissues.
Orchestrating Immune Responses
Once activated, helper T cells communicate within the immune system by releasing signaling molecules called cytokines. These cytokines act as chemical messengers, directing and enhancing the activities of other immune cells to mount a coordinated attack against pathogens. Helper T cells do not directly eliminate infected cells or pathogens; instead, they empower other immune components to do so.
One of their primary roles is activating B cells, which are responsible for producing antibodies. Activated helper T cells provide signals, including cytokines like IL-4, that enable B cells to proliferate and differentiate into plasma cells. These plasma cells then secrete specific antibodies against the identified antigen. Helper T cells also enhance the killing capabilities of cytotoxic T cells, which destroy infected host cells. They boost the effectiveness of macrophages, enabling these cells to efficiently engulf and digest microbes, and recruit other immune cells to the site of infection, ensuring a strong and sustained immune response.
Diverse Roles of Helper T Cell Subtypes
Helper T cells differentiate into distinct subtypes, each specializing in combating different types of threats and tailoring the immune response. This specialization allows the immune system to respond precisely to the nature of the invading pathogen. The environment, particularly the specific cytokines present during their activation, dictates which subtype a helper T cell will become.
Th1 cells are involved in clearing intracellular pathogens like viruses and certain bacteria. They achieve this by producing cytokines such as interferon-gamma (IFN-γ) and interleukin-2 (IL-2), which activate macrophages to destroy microbes residing inside them and enhance the activity of cytotoxic T cells. In contrast, Th2 cells respond to extracellular parasites, such as helminths, and are associated with allergic reactions. They secrete cytokines like IL-4, IL-5, and IL-13, which promote antibody production by B cells and activate other immune cells like eosinophils.
Th17 cells defend against extracellular bacteria and fungi and are involved in inflammatory responses. They produce cytokines such as IL-17 and IL-22, which recruit neutrophils, a type of white blood cell, to sites of infection and help maintain barrier integrity.
Finally, regulatory T cells (Tregs) maintain immune balance and prevent autoimmunity. Tregs produce immunosuppressive cytokines like IL-10 and TGF-β, which dampen immune responses and prevent the immune system from attacking the body’s own tissues.