DurgicalDurgical
EN
Open the app

Immunology · Immune 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

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.

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)

Microorganisms

← All of 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 (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); CD4 Lymphocyte Count (MedlinePlus, U.S. National Library of Medicine, 2024); Blood differential test (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.