Question
CD4 lymphocyte.
Answer
CD4 lymphocytes are a subset of T lymphocytes (“T-helper cells”) that express the CD4 surface glycoprotein, which recognizes antigenic peptides presented on MHC class II molecules by antigen-presenting cells, and whose principal role is to orchestrate and amplify the immune response by secreting cytokines and providing help to other immune cells (B cells, cytotoxic T cells, macrophages), 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, important against helminths and in allergy), Th17 (secretes IL-17, recruits neutrophils, important against extracellular bacteria/fungi), and T follicular helper cells (support B-cell antibody responses in germinal centres).
Clinical significance: CD4 lymphocytes are the primary target of HIV, which uses the CD4 molecule (along 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, and guide the timing of prophylaxis (e.g., against Pneumocystis jirovecii pneumonia below 200 cells/μL, and against disseminated Mycobacterium avium complex below 50 cells/μL) and initiation/monitoring of antiretroviral therapy.