Question
CD4+ T lymphocytes
Answer
CD4+ T lymphocytes (“T-helper cells”) are a subset of T lymphocytes that express the CD4 surface glycoprotein, which recognizes antigenic peptides presented on MHC class II molecules by antigen-presenting cells; their principal role is to orchestrate and amplify the immune response by secreting cytokines and providing help to other immune cells, rather than directly killing target cells.
Functional subsets: Th1 (secretes IFN-γ, activates macrophages, drives cell-mediated immunity against intracellular pathogens), Th2 (secretes IL-4/IL-5/IL-13, promotes antibody production and eosinophil activation), Th17 (secretes IL-17, recruits neutrophils), and T follicular helper cells (support B-cell antibody responses in germinal centres).
Clinical significance: CD4+ T cells are the primary target of HIV, which uses the CD4 molecule (with a chemokine co-receptor, CCR5 or CXCR4) as its entry receptor; progressive depletion of CD4+ T cells is the hallmark of AIDS-related immunodeficiency, and the CD4 count is used clinically to stage HIV disease, predict risk of specific opportunistic infections (e.g., Pneumocystis jirovecii pneumonia below 200 cells/μL), and guide antiretroviral therapy and prophylaxis decisions.