Minor population: γδ T lymphocytes
This group of cells accounts for only 5% of the total T lymphocyte pool. Their main feature is specialization in specific functions depending on the expression of CD4 and CD8 markers. According to this criterion, they can be divided into three subgroups:
- Double-negative (CD4⁻CD8⁻). These cells lack both markers. Their primary localization is the mucosal epithelium and skin. They act as the first line of defense of the body (perform a barrier function) and possess a unique ability to recognize non-protein antigens.
- Single-positive (CD4⁻CD8⁺). These cells carry only the CD8 marker on their surface. They are characterized by pronounced cytotoxic activity, i.e., the ability to directly destroy target cells.
- Single-positive (CD4⁺CD8⁻). They bear only the CD4 marker. To date, the exact physiological function of this specific subgroup has not been definitively established.
Major population: αβ T lymphocytes
These are the main workforce of cellular immunity, accounting for 95% of all T cells. This entire extensive population is strictly divided according to two main criteria: the current stage of maturation and the final functional role.
The developmental process of these cells is inextricably linked with the central organ of immunogenesis. Immature forms of αβ T lymphocytes are found only in the thymus. At the maturation stage, they pass through specific stages of surface marker expression:
- Initially, they are double-negative cells (CD4⁻CD8⁻).
- Then they evolve into double-positive forms, simultaneously expressing both markers (CD4⁺CD8⁺).
Mature effector forms of αβ T lymphocytes
Upon leaving the thymus and completing the maturation process, αβ T lymphocytes lose one of the markers and become mature effector cells. They are divided into two fundamentally different functional groups:
T helper cells (CD4⁺CD8⁻) Cells carrying the CD4 marker act as coordinators of the immune response. Depending on the type of signals they secrete, they are subdivided into:
- Th1: responsible for the activation of cellular immunity.
- Th2: specialize in the activation of the humoral immune response.
- Other subpopulations: engage in general immunoregulation.
Cytotoxic T lymphocytes (CD4⁻CD8⁺) Cells with the CD8 marker are the main effectors that directly form cellular immunity.
- Main focus: providing potent antiviral and antitumor defense of the body.
- Involvement in pathology: cytotoxic lymphocytes are the key link in undesired graft rejection reactions and are also actively involved in destructive autoimmune processes.
Briefly: answers to topical questions
What subpopulations are T helper cells divided into, and what is the function of each?
T helper cells are subdivided into Th1, Th2, and other regulatory cell subpopulations.
- Th1 — responsible for the activation of cellular immunity (type I immune inflammation), macrophage activation, and stimulation of cytotoxic CD8+ T lymphocytes.
- Th2 — responsible for the activation of humoral immunity, providing immunoglobulin isotype switching for the synthesis of antibodies of different classes.
- Others — carry out immunoregulation.
By means of what effector molecules do cytotoxic T lymphocytes destroy target cells?
Cytotoxic T lymphocytes destroy target cells using perforins, granzymes, and the Fas ligand.
- Perforins — proteins that insert into the target cell membrane and form pores in it.
- Granzymes — serine proteases that penetrate into the cell through the formed pores and trigger apoptosis.
- Fas ligand (FasL) — a molecule on the surface of CTLs that interacts with the Fas receptor (CD95) on the target cell, which also leads to the triggering of apoptosis.
What main cytokines are produced by Th1 and Th2 lymphocytes?
Th1 and Th2 lymphocyte subpopulations produce various cytokines.
- Th1 lymphocytes — secrete interferon-gamma (IFN-γ), interleukin-2 (IL-2), tumor necrosis factor (TNF), and granulocyte-macrophage colony-stimulating factor (GM-CSF).
- Th2 lymphocytes — produce interleukins IL-4, IL-5, IL-6, IL-10, and IL-13.
In which organs does the antigen-dependent stage of T lymphocyte differentiation take place?
The antigen-dependent stage of T lymphocyte differentiation takes place in peripheral (secondary) lymphoid organs and tissues. These include:
- Lymph nodes;
- Spleen;
- Mucosa-associated lymphoid tissue (MALT), including the palatine tonsils, solitary follicles (Peyer's patches) of the small intestine, and the lymphoid apparatus of the vermiform appendix.
In these organs, naive T lymphocytes encounter antigens and undergo subsequent specialization.
How do γδ T lymphocytes differ from αβ T lymphocytes in their mechanism of antigen recognition?
γδ T lymphocytes recognize non-protein antigens. For αβ T lymphocytes, sources indicate the recognition of an antigenic peptide as part of a complex with an MHC molecule: CD8+ T cells recognize the peptide in the context of MHC-I, and CD4+ T cells in the context of MHC-II.
To avoid confusing the markers of mature effector cells: "Helpers" are always CD4+ (they regulate immunity). "Cytotoxic" cells (killers) are always CD8+ (they destroy viruses and tumors).
Class and exam questions
Where are double-negative γδ T lymphocytes localized, and what is their role?
They are located in the epithelium of the skin and mucous membranes. Their main task is to provide the first barrier line of defense and recognize non-protein antigens.
In which organ are immature forms of αβ T lymphocytes located?
Immature forms of αβ cells (both double-negative CD4⁻CD8⁻ and double-positive CD4⁺CD8⁺) are found exclusively in the thymus.
Which cells are responsible for activating humoral immunity?
A specific subpopulation of mature T helpers — Th2 (CD4⁺CD8⁻) — is responsible for activating the humoral arm of immunity.
What is the role of cytotoxic T lymphocytes in pathological processes?
Cytotoxic cells (CD8+) take a direct part in the development of autoimmune processes and transplanted graft rejection reactions.
What else to study on this topic
- Mechanisms of non-protein antigen recognition by γδ lymphocytes
- The process of cell maturation in the thymus (transition from double-negative to double-positive)
- Interaction of Th1 and Th2 in the formation of the immune response
- The role of CD8+ cells in graft rejection
- Cytotoxic activity of single-positive γδ T lymphocytes
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