[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Immunology](https://durgical.com/en/immunology/) › [Immune cells](https://durgical.com/en/immunology/#a-cells) › CD4⁺ helper T cell [Immunology](https://durgical.com/en/immunology/) · [Immune cells](https://durgical.com/en/immunology/#a-cells) # CD4⁺ helper T cell - Cell Helper T cells are the coordinators of the immune response: they recognize antigen on MHC class II and tell B cells, macrophages and CD8 cells what to do. Updated 2026-11-01 ## Key points - CD4 binds MHC class II; the T-cell receptor sits in a complex with CD3. - Activation needs signal 1 (TCR with peptide-MHC II) and signal 2 (CD28 with B7, CD80/CD86); without signal 2 the cell becomes anergic. - Subsets: Th1 (interferon-γ, macrophages), Th2 (IL-4, IL-5, IL-13, allergy and helminths), Th17 (IL-17, neutrophils), Tfh (B-cell help in germinal centers) and Treg. - CD40 ligand on activated CD4 cells binding CD40 on B cells is needed for class switching; its absence causes hyper-IgM syndrome. - HIV enters through CD4 plus a coreceptor (CCR5 or CXCR4); a CD4 count of 200 cells/mm³ or less defines AIDS. ## What it is CD4⁺ T cells recognize peptides presented on MHC class II by antigen-presenting cells. Once activated, they secrete cytokines that steer the response: they help B cells make high-affinity, class-switched antibody, activate macrophages and support CD8 T cells. HIV infects and destroys them, which is why the CD4 count measures how much the virus has weakened immunity. ## Key facts - Origin: lymphoid progenitor in the marrow, selected in the thymus. - Naive cells are long-lived and recirculate; memory cells can persist for years. - They are the majority of blood T cells in health; HIV inverts the usual CD4:CD8 ratio. ## Function - Cytokine help for B cells (antibody, class switching, affinity maturation). - Macrophage activation against intracellular microbes such as mycobacteria. - Help for CD8 T-cell responses and memory. - Recruitment of neutrophils (Th17) or eosinophils (Th2). ## Markers - CD3 - CD4 - TCR αβ - CD28 - CD40L (CD154) when activated ## Clinical notes - HIV: falling CD4 count brings opportunistic infections such as Pneumocystis pneumonia, tuberculosis, toxoplasmosis and cryptococcal meningitis. - Tuberculosis and HIV coinfection is a major problem in Latin America; each worsens the other. - A Th1 cytokine defect (IL-12 or interferon-γ axis) leads to severe mycobacterial and Salmonella infections. - Superantigens (for example toxic shock toxin) bind MHC II and the TCR outside the groove and activate many CD4 cells at once. ## In the lab - Absolute CD4 count and percentage by flow cytometry, used with viral load in HIV. - Low CD4 counts also occur after chemotherapy, with immunosuppressive drugs and in some infections. ## Memory aid The helper is a coach with a two-page playbook: CD4 reads class II on the antigen-presenting cell and then calls the plays for everyone else. ## Test yourself Think of the answer, then tap the question to see it. ## Which MHC class do CD4 T cells recognize? MHC class II, found on antigen-presenting cells. ## What happens if a naive CD4 T cell gets signal 1 without signal 2? It becomes anergic (unresponsive) instead of activated. ## Which interaction is needed for B-cell class switching, and what disease follows its absence? CD40 ligand on the helper T cell binding CD40 on the B cell; its absence causes hyper-IgM syndrome. ## Which CD4 count defines AIDS in a person with HIV? A count of 200 cells per cubic millimeter or less. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/immunology/cd4-helper-t-cell/) ## Related - [Th1, Th2, Th17, Tfh and Treg](https://durgical.com/en/immunology/th1-th2-th17-tfh-and-treg/) - [T-cell activation: two signals and superantigens](https://durgical.com/en/immunology/t-cell-activation-two-signals-and-superantigens/) - [MHC/HLA class I and II](https://durgical.com/en/immunology/mhc-hla-class-i-and-ii/) - [Dendritic cell (professional antigen-presenting cell)](https://durgical.com/en/immunology/dendritic-cell/) - [B cell and plasma cell](https://durgical.com/en/immunology/b-cell-and-plasma-cell/) - [Regulatory T cell (Treg, FOXP3)](https://durgical.com/en/immunology/regulatory-t-cell/) - [Flow cytometry and the CD4 count](https://durgical.com/en/immunology/flow-cytometry-and-the-cd4-count/) ## Related conditions - [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/) - [Tuberculosis](https://durgical.com/en/conditions/tuberculosis/) ## Microorganisms - [Human immunodeficiency virus](https://durgical.com/en/microbiology/human-immunodeficiency-virus/) [← All of immunology](https://durgical.com/en/immunology/) Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given. Sources: [21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types](https://openstax.org/books/anatomy-and-physiology-2e/pages/21-3-the-adaptive-immune-response-t-lymphocytes-and-their-functional-types) (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); [CD4 Lymphocyte Count](https://medlineplus.gov/lab-tests/cd4-lymphocyte-count/) (MedlinePlus, U.S. National Library of Medicine, 2024); [Blood differential test](https://medlineplus.gov/ency/article/003657.htm) (MedlinePlus, U.S. National Library of Medicine, 2024). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/immunology/cd4-helper-t-cell/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/cd4-helper-t-cell/