CD4+ T cells, or helper T cells, are one type of lymphocyte that helps coordinate the immune response against infection and disease. They interact and activate other cells in the immune system.
The immune system is incredibly complex and contains many different components. Cells that make up the immune system include lymphocytes, macrophages, neutrophils, and dendritic cells. These cells target pathogens — disease-causing agents — and other foreign substances.
T cells are lymphocytes, and there are three distinct classes of them:
- cytotoxic CD8+ T cells
- helper CD4+ T cells
- regulatory CD4+ T cells
The type of protein on the surface of the T cell differentiates them. This article explores the role of helper T cells in health and disease.
While they have no direct involvement in neutralizing foreign substances, CD4+ T cells have various functions that help produce a vigorous immune response. Though various subsets of them exist, they primarily activate other immune cells.
Some CD4+ T-cells can become activated and form memory T cells. These are important for the body to maintain long-term immunity against infection.
Measuring a person’s CD4+ T cell may help diagnose HIV. This virus directly invades CD4+ T cells and replicates within them. As the virus replicates, it slowly destroys CD4+ T cells, weakening the immune system’s ability to fight off other infections.
There are several types of CD4+ T cells, each with different functions. The
- T helper cell 1 (TH1): These coordinate the immune responses against pathogens within the cell, for example, bacteria. TH1 produces cytokines, proteins that can act on the immune system or directly destroy pathogens. Examples of cytokines that TH1 produces include:
- interleukin 2
- tumor necrosis factor beta
- T helper cell 2 (TH2): They coordinate immune responses against pathogens outside the body’s cells, for example, helminths, which are parasitic worms. TH2 also produces cytokines, which activate other lymphocytes and regulate allergic reactions. Examples of cytokines TH2 that produces include:
- interleukin 4
- interleukin 5
- interleukin 13
- T helper cell 17 (TH17): They produce interleukin 17, a signaling molecule that activates other immune cells. TH17 helps protect the body from extracellular bacteria and fungi present outside the body’s cells.
- Regulatory T cells (Treg): These regulatory T cells monitor and suppress other immune responses, helping prevent autoimmune issues and maintain homeostasis.
Memory CD4+ T-cells are beneficial to maintaining a person’s immunity. They evolve from naive CD4+ T-cells, which are precursors to memory cells, and can recognize and target germs the body has encountered before. However, the second time the body encounters the germ, it produces a faster and stronger response.
How does the body produce them?
A germ — or antigen — must activate naive CD4+ T cells to start the transformation. The activation happens when a naive CD4+ T cell interacts with another immune cell, known as an antigen-presenting cell (APC).
APCs sit within the lymph nodes. They are specialized cells with the ability to recognize and process germs. CD4+ T-cells can easily recognize antigens as atypical and become activated.
Once they activate, the naive CD4+ T cells divide, forming multiple clones. Some of these clones become memory cells. They can produce cytokines that activate other immune cells, whereas naive CD4+ T cells cannot.
Typically, memory CD4+ T cells remain dormant in the body until they recognize a previously encountered germ.
HIV targets CD4+ T cells. When the virus
Measuring CD4+ T cells circulating in the blood provides valuable information about how well the immune system functions, especially in people with HIV.
Because HIV destroys CD4+ T cells, healthcare professionals can use CD4 cell levels to monitor HIV progression and AIDS. A typical CD4 cell count is between
When a person’s CD4 count
A doctor may prescribe antiretroviral therapy (ART) to slow the progression of HIV and keep CD4 levels stable. ART targets different steps in the HIV replication cycle, inhibiting or stopping its spread within the body.
- the proteins present on their surface
- their response to pathogens
- their function
CD4+ T cells can trigger and coordinate the immune response. They recognize pathogens and secrete cytokines that activate other immune cells, including CD8+ T cells. However, they have no direct involvement in attacking and neutralizing pathogens.
In contrast, CD8+ T cells directly attack pathogens and cancerous cells by binding to them and secreting toxic molecules that stop pathogens from replicating and cause cell death. They are also known as cytotoxic T-cells or cytotoxic lymphocytes.
CD4+ T cells are lymphocytes that have various functions critical to protecting the human body from infection and other harmful antigens. Several types exist, including TH1, TH2, TH17, and Treg cells, each with slightly different roles and functions. However, they all work together to help protect the body from foreign pathogens.
CD4+ T cells primarily activate other immune cells, including CD8+ T cells. Some CD4+ T cells become memory cells after encountering a pathogen and becoming activated. Memory cells help the body recognize and respond more quickly in future encounters with the same invader.
They may also be the target of some pathogens, for example, HIV. Therefore, healthcare professionals can use CD4 levels to monitor HIV progression and AIDS.