Question
T-helper cells.
Answer
T-helper (Th) cells are a subset of T lymphocytes characterized by expression of the CD4 surface marker, which recognizes antigenic peptides presented on MHC class II molecules by antigen-presenting cells (dendritic cells, macrophages, B cells). Rather than directly killing target cells, their principal role is to orchestrate and amplify the immune response by secreting cytokines and providing help to other immune cells.
Functional subsets (differentiated based on the cytokine milieu during activation):
- Th1 cells — driven by IL-12; secrete IFN-γ, activating macrophages and promoting cell-mediated immunity, particularly important against intracellular pathogens (e.g., M. tuberculosis).
- Th2 cells — driven by IL-4; secrete IL-4, IL-5, IL-13, promoting B-cell antibody production (particularly IgE) and eosinophil activation — important in immunity against helminths and in allergic disease.
- Th17 cells — driven by IL-6/TGF-β/IL-23; secrete IL-17, recruiting neutrophils, important in mucosal defence against extracellular bacteria/fungi.
- T follicular helper (Tfh) cells — provide help to B cells within germinal centres, essential for antibody class-switching and affinity maturation.
- Regulatory T cells (Treg, though sometimes classified separately) — CD4+CD25+FoxP3+; suppress excessive immune responses and maintain self-tolerance.
Clinical significance: CD4+ T-helper cells are the primary target of HIV, which uses the CD4 molecule (along with a chemokine co-receptor) as its entry receptor; progressive depletion of CD4+ T-helper cells is the basis of AIDS-related immunodeficiency, and CD4 count is used clinically to stage HIV disease and guide opportunistic-infection prophylaxis.