Maturation and Actions of Subsets of Helper T-Cells

Overview of T Lymphocytes

Three main classes of T cells:
- Cytotoxic T cells: Directly kill infected host cells.
- T regulatory cells: Suppress the immune response by inhibiting helper T cells, dendritic cells, and cytotoxic T cells.
- Helper T cells: Activate other immune cells such as macrophages, dendritic cells, cytotoxic T cells, and B cells.

**Naive Helper T-Cell Activation: ** Occurs when encountering a presented antigen in peripheral lymphoid tissue.
Subtypes of Effector Helper T Cells: Determined by cytokines released by the antigen-presenting cell (APC).
**Subtypes: ** T helper 1 (Th1), T helper 2 (Th2), T helper 17 (Th17).

Maturation: Triggered by cytokine IL-12 from APC.
**Functions: **
- Secretes interferon-gamma (IFN-γ), activating macrophages.
- Causes B cells to produce IgG antibodies.
- IgG is crucial for activating the classical complement pathway and opsonization of microbes.
- Activates cytotoxic T cells.
Role in Immunity: Protects against intracellular microbes, including mycobacteria.
Overactivation: Involved in type 4 hypersensitivity reactions.

Maturation: Triggered by cytokine IL-4 from mast cells.
Functions:
- Secretes IL-4 to stimulate B cells to secrete IgE antibodies.
- Produces IL-5, activating eosinophils.
- Produces IL-13, stimulating mucosal epithelial cells to produce mucus.
Role in Immunity: Targets large multicellular pathogens like parasites (including helminths).
Allergies + Atopy: Predominant in allergic responses, atopic individuals show higher Th2 to Th1 ratio, resulting in higher IgE levels.
Overactivation: Involved in type 1 hypersensitivity.

Maturation: Triggered by cytokines IL-6 and TGF-beta.
Functions:
- Produces IL-17, recruiting phagocytic cells like neutrophils and macrophages to infected tissues.
Role in Immunity: Defense against certain extracellular bacteria and fungi.
Dysregulation: Linked to autoimmune and inflammatory disorders such as inflammatory bowel disease, multiple sclerosis, and psoriasis.