Helper T cells are critical for adaptive immunity, orchestrating and enhancing immune responses. They don't directly kill infected cells or pathogens but instead, activate other immune cells to do so.
Here's a breakdown of how helper T cells work:
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Activation: Helper T cells become activated when they encounter an antigen-presenting cell (APC), such as a macrophage, dendritic cell, or B cell, that displays a specific antigen (a fragment of a pathogen) bound to a Major Histocompatibility Complex (MHC) class II molecule on its surface.
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Co-stimulation: In addition to antigen presentation via MHC II, co-stimulatory molecules on the APC must bind to co-stimulatory receptors on the T cell for full activation. This prevents T cell activation from occurring inappropriately.
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Cytokine Secretion: Once activated, helper T cells secrete various cytokines, which are signaling molecules that influence the behavior of other immune cells. The specific cytokines secreted determine the type of immune response that develops.
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Differentiation: Activated helper T cells differentiate into distinct subsets, each tailored to combat different types of infections. The major subsets include:
- TH1 cells: Produce interferon-gamma (IFN-γ), which activates macrophages to kill intracellular pathogens and promotes cell-mediated immunity.
- TH2 cells: Produce IL-4, IL-5, and IL-13, which promote humoral immunity by activating B cells to produce antibodies, particularly IgE. These cells are also important in defense against parasites.
- TH17 cells: Produce IL-17, which promotes inflammation and recruits neutrophils to the site of infection, important in defense against extracellular bacteria and fungi.
- Treg cells: Produce immunosuppressive cytokines such as TGF-β and IL-10, which helps to control the immune response and prevent autoimmunity.
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Helping other Immune Cells:
- B cell Activation: Helper T cells help B cells produce antibodies by binding to the B cell and releasing cytokines that stimulate B cell proliferation, differentiation into plasma cells (antibody-secreting cells), and antibody class switching. This process is essential for generating high-affinity antibodies that can effectively neutralize pathogens.
- Macrophage Activation: Helper T cells, specifically TH1 cells, activate macrophages to destroy ingested microbes. This is crucial for eliminating intracellular pathogens that can survive within macrophages.
- Cytotoxic T cell (CTL) Activation: Helper T cells also help activate cytotoxic T cells to kill infected target cells. This involves cytokine secretion and direct interaction between helper T cells and CTLs, promoting their proliferation and differentiation into fully functional killer cells.
| Helper T Cell Function | Immune Cell Activated | Mechanism |
|---|---|---|
| Activate B cells to secrete antibodies | B cells | Binds to B cell and releases cytokines (e.g., IL-4, IL-5, IL-6) that stimulate B cell proliferation, differentiation into plasma cells, and antibody class switching. |
| Activate macrophages to destroy ingested microbes | Macrophages | Produces interferon-gamma (IFN-γ) that enhances macrophage activity and intracellular killing mechanisms. |
| Activate cytotoxic T cells (CTLs) | CTLs | Cytokine secretion (e.g., IL-2) and direct interaction to promote CTL proliferation and differentiation into killer cells. |
In summary, helper T cells play a central role in adaptive immunity by coordinating and amplifying the responses of other immune cells, ensuring effective clearance of pathogens. As stated in the reference, they are "arguably the most important cells in adaptive immunity, as they are required for almost all adaptive immune responses."
