T cells, also known as T lymphocytes, are a type of white blood cell that plays a central role in cell-mediated immunity. They derive their name from the fact that they mature in the thymus gland. There are several different types of T cells, each with different functions:
Cytotoxic T Cells
Cytotoxic T cells (also known as CD8+ T cells) directly kill cells infected with viruses, intracellular bacteria, and cancer cells. They work by releasing cytotoxic granules that contain chemicals that induce apoptosis (programmed cell death) in the target cell. The main cytotoxic chemicals contained in these granules are:
- Perforin – creates pores in the target cell membrane
- Granzymes – enter the target cell through the pores and cleave proteins to trigger apoptosis
- Granulysin – damages the mitochondria of the target cell
So in summary, yes cytotoxic T cells do release chemicals that directly induce programmed cell death in infected/cancerous cells.
Helper T Cells
Helper T cells (also known as CD4+ T cells) do not directly kill infected cells. Rather, they help stimulate and direct other immune cells. They recognize infected cells and secrete cytokines that activate cytotoxic T cells, B cells, macrophages and other cell types. Some key cytokines released by helper T cells include:
- Interleukin-2 – stimulates growth of cytotoxic T cells
- Interferon-gamma – activates macrophages
- Interleukin-4 – stimulates B cell activation and antibody production
So while helper T cells release cytokines, they do not directly release cytotoxic chemicals to kill cells.
Regulatory T Cells
Regulatory T cells (also known as suppressor T cells) modulate the immune system and prevent autoimmune reactions. They accomplish this by secreting anti-inflammatory cytokines such as:
- Interleukin-10 – inhibits cytokine production by other T cells
- Transforming growth factor-beta – suppresses activation of other immune cells
Like helper T cells, regulatory T cells release cytokines that indirect suppress the activity of other immune cells, rather than directly killing cells.
Natural Killer T Cells
Natural killer T cells (NKT cells) bridge the innate and adaptive immune systems. They can directly kill infected cells similarly to cytotoxic T cells by releasing perforin and granzymes. However, NKT cells are also able to rapidly secrete cytokines to stimulate other immune cells. Overall, NKT cells have properties of both cytotoxic T cells and helper T cells.
- Cytotoxic T cells directly kill infected/cancerous cells by releasing cytotoxic granules containing perforin, granzymes and other chemicals.
- Helper T cells, regulatory T cells, and some other T cell types do not directly kill cells, but rather activate/modulate other immune cells by secreting cytokines.
- Natural killer T cells have properties of both cytotoxic T cells and helper T cells – they can directly kill cells as well as secrete cytokines.
So in short, the main type of T cell that directly releases cytotoxic chemicals to kill cells are the cytotoxic T cells. Other types like helper and regulatory T cells have indirect roles through cytokine secretion.
|T Cell Type||Directly Kills Infected Cells?||Main Mechanism|
|Cytotoxic T Cell||Yes||Release cytotoxic granules containing perforin, granzymes, etc.|
|Helper T Cell||No||Secrete cytokines like IL-2, IFN-gamma to stimulate other immune cells|
|Regulatory T Cell||No||Secrete cytokines like IL-10, TGF-beta to suppress immune cells|
|Natural Killer T Cell||Yes||Release cytotoxic granules and secrete cytokines|
- Chaplin, D. (2010). Overview of the immune response. Journal of Allergy and Clinical Immunology, 125(2), S3-S23.
- Goronzy, J. & Weyand, C. (2019). Immune aging and autoimmunity. Cellular and Molecular Life Sciences, 76(9), 1615–1623.
- Orange, J.S. (2013). Natural killer cell deficiency. Journal of Allergy and Clinical Immunology, 132(3), 515–526.