Helper T Cells (CD4+): Orchestrators of Adaptive Immunity

CD4⁺ T helper (Th) cells are central coordinators of adaptive immunity.

Activation

TCR recognizes peptide-MHC II on APCs (dendritic cells, macrophages, B cells) + costimulation (CD28-B7) → clonal expansion.

Subsets & Cytokines

• Th1: IFN-γ, IL-2 → macrophage activation, CTL responses (intracellular pathogens).
• Th2: IL-4, IL-5, IL-13 → B cell class-switching (IgE, IgG1), eosinophils (helminths, allergy).
• Th17: IL-17, IL-22 → neutrophil recruitment, mucosal defense (fungi, extracellular bacteria).
• Tfh: IL-21, CXCL13 → germinal center B cell help, affinity maturation.
• Treg: IL-10, TGF-β → suppression, tolerance.

Key Functions

1. Activate CTLs (via IL-2, CD40L).
2. Drive B cell differentiation → antibodies.
3. Macrophage activation (Th1).
4. Recruit/activate granulocytes (Th17).
5. Form memory T cells for rapid recall.

Clinical Relevance

HIV depletes CD4⁺ cells → loss of coordination → opportunistic infections. Autoimmunity involves Th1/Th17 dysregulation. Cancer immunotherapy aims to enhance Th1/Tfh anti-tumor responses.