T cells and killer T cells are both types of lymphocytes, or white blood cells, that play important roles in the immune system. However, they have some key differences in terms of their functions and characteristics.
Quick Answer
The main differences between T cells and killer T cells are:
- T cells are a broad group of lymphocytes that coordinate the immune response, whereas killer T cells are a specific type of cytotoxic T lymphocyte that can directly kill infected or cancerous cells.
- Most T cells recognize antigens on infected cells and signal other immune cells, while killer T cells release toxins to induce infected cell apoptosis.
- T cells require activation before performing their functions, but some killer T cells can recognize antigens and kill cells without activation.
- Common types of T cells include helper, regulatory, memory, and natural killer T cells, while killer T cells typically refer to either CD8+ cytotoxic T cells or natural killer T cells.
Origin and Development
T cells and killer T cells originate from the same type of hematopoietic stem cells in the bone marrow. These stem cells differentiate into lymphoid progenitor cells, which further mature into T cell precursors that travel to the thymus gland. In the thymus, T cell precursors undergo positive and negative selection, processes that establish central tolerance and prevent autoimmunity.
During positive selection, T cells that interact well with self MHC molecules receive survival signals, while those that fail this interaction die by apoptosis. Next, T cells undergo negative selection, which destroys T cells that bind too strongly to self antigens presented by antigen presenting cells. This prevents autoreactive T cells from moving to peripheral tissues.
After selection, T cells exit the thymus as mature naive T cells that await activation by antigens. Most T cells express T cell receptors (TCRs) with CD4 or CD8 co-receptors. T cells become effector T cells upon activation by antigens presenting by MHC molecules on antigen presenting cells (APCs).
Killer T cells, also called cytotoxic T lymphocytes (CTLs), undergo the same development process in the bone marrow and thymus. However, they differentiate specifically into CD8+ T cells that have the ability to directly induce cell death in infected or dysfunctional cells. Activated CD8+ T cells differentiate into cytotoxic effector T cells that travel throughout the body to identify and kill these abnormal cells.
Mechanisms of Action
Most T cells function by recognizing processed antigen peptides presented on MHC molecules. When the TCR binds an antigen, the T cell becomes activated if it also receives co-stimulation from the APC. Activated helper T cells (TH cells) release cytokines that stimulate growth, activation and differentiation of other immune cells like B cells, macrophages and cytotoxic T cells.
Regulatory T cells (Tregs) dampen the immune response by releasing immunosuppressive cytokines. Memory T cells remain after an infection is cleared and allow faster pathogen clearance upon re-exposure.
Natural killer T (NKT) cells share properties of both T cells and natural killer cells. They recognize lipid antigens and can rapidly produce cytokines to stimulate the immune response.
In contrast, killer T cells directly recognize antigens on infected cells via their TCR. Most killer T cells are CD8+ and are activated by interacting with antigen-MHC class I molecules. When activated, they release perforin and granzymes, proteins that form pores in the target cell membrane. This allows entry of granzymes which trigger apoptosis, or programmed cell death.
This cytotoxic mechanism directly kills infected or cancerous cells. Killer T cells also express Fas ligand that can bind the Fas receptor on target cells and induce apoptotic cell death. Furthermore, release of interferon-gamma and TNF-alpha by killer T cells can inhibit viral replication and promote inflammatory responses against pathogens.
Cell Surface Markers
T cells are characterized by the presence of T cell receptors on their cell surface. However, different T cell subtypes have different surface markers:
| T cell Subtype | Surface Markers |
|---|---|
| Helper T cells | CD3, CD4, TCR |
| Regulatory T cells | CD4, CD25, FOXP3, CTLA-4 |
| Memory T cells | CD3, CD4/CD8, CD45RO |
| Natural killer T cells | CD3, CD56, CD16, NK1.1 |
| Killer T cells | CD3, CD8, TCR |
All T cells express the T cell receptor (TCR) and CD3. Helper T cells involved in activating other immune cells typically express CD4, while killer T cells that directly kill infected cells express CD8.
Regulatory T cells that suppress immune responses constitutively express CD25 and FOXP3. Memory T cells can be distinguished by CD45RO. Natural killer T cells express markers like CD56 and CD16 characteristic of natural killer cells.
Activation Requirements
Most T cell subtypes require activation before carrying out their immune functions. Activation occurs when the TCR binds a specific antigen peptide and the T cell receives co-stimulatory signals from nearby APCs.
Common co-stimulatory molecule pairs include CD80/CD86 on APCs binding CD28 on T cells, as well as CD40L on T cells binding CD40 on APCs. Without proper co-stimulation, T cells enter anergy and cannot respond effectively.
However, some T cells can respond without secondary co-stimulation. For example, CD8+ killer T cells activated through high affinity TCR-antigen binding undergo autonomous activation. Memory T cells also require less co-stimulation for reactivation upon antigen re-encounter.
Natural killer T cells recognize lipid antigens on CD1d of antigen presenting cells, allowing swift cytokine secretion without co-stimulation. Thus, certain T cell subtypes can immediately execute effector functions when stimulated.
Roles in Immune Response
Overall, T cells orchestrate immune responses against infected or dysfunctional cells in the body. Different types of T cells play unique roles:
- Helper T cells: Stimulate growth and differentiation of B cells, cytotoxic T cells, macrophages
- Regulatory T cells: Suppress excessive immune reactions and prevent autoimmunity
- Memory T cells: Provide long-term protection against re-infection
- Natural killer T cells: Rapidly produce cytokines upon lipid antigen recognition
- Killer T cells: Directly kill infected cells and cancerous cells
Killer T cells specifically function to induce apoptosis in cells presenting foreign antigens, like viruses and intracellular bacteria, or abnormal self antigens, as in cancerous cells. This allows direct elimination of pathogens and tumors.
Thus, most T cells work together to coordinate appropriate immune responses, while killer T cells provide immediate cytotoxity to eradicate unhealthy cells that could harm the body. Both general T cells and specialized killer T cells are critical for immune protection.
Conclusion
In summary, T cells and killer T cells arise from a common lymphoid lineage but perform distinct roles in cell-mediated immunity. Conventional T cells recognize antigens and secrete cytokines to regulate immune responses. In contrast, cytotoxic killer T cells directly recognize and kill infected, cancerous, or otherwise abnormal cells in the body.
While T cells require activation for most of their functions, some killer T cells like CD8+ CTLs and NKT cells can immediately kill target cells following TCR antigen recognition. However, both T cells and killer T cells work synergistically to provide immune protection against pathogens and tumors.