helper T cell

CD4+ helper T cells are activated when their T-cell receptor (TCR) recognizes a peptide antigen presented on MHC class II molecules by professional APCs (dendritic cells, macrophages, B cells), along with costimulatory signal via CD28 binding B7 (CD80/86) on the APC. Once activated, naive CD4+ T cells differentiate into distinct subsets based on cytokine milieu: Th1 cells (driven by IL-12, IFN-gamma) activate macrophages and cytotoxic T cells, important for intracellular pathogen defense; Th2 cells (driven by IL-4) promote B cell class switching (IgE) and are involved in allergic responses and defense against helminths; Th17 cells (driven by IL-6, TGF-beta, IL-23) recruit neutrophils and defend against extracellular bacteria and fungi, and are implicated in autoimmune disease; and regulatory T cells (Tregs, marked by FOXP3, CD25) suppress immune responses and maintain self-tolerance. Helper T cells secrete cytokines (e.g., IL-2 for T cell proliferation, IFN-gamma for macrophage activation) and provide CD40L-CD40 interaction with B cells to induce immunoglobulin class switching and germinal center formation. Clinically, CD4+ T cells are the primary target of HIV via the CD4 receptor and CXCR4/CCR5 co-receptors; progressive depletion leads to AIDS-defining opportunistic infections when CD4 count falls below specific thresholds (e.g., <200 cells/microL for Pneumocystis jirovecii pneumonia, <100 for Toxoplasma and cryptococcal infections, <50 for CMV and Mycobacterium avium complex). DiGeorge syndrome results in thymic aplasia and T-cell deficiency due to failure of pharyngeal pouch development. Understanding the Th1/Th2/Th17/Treg paradigm is essential for understanding autoimmune diseases, allergy, and vaccine design (e.g., adjuvants that skew toward Th1 vs Th2).