Model Answer
0 min readIntroduction
Immunity is the ability of an organism to resist harmful pathogens, including bacteria, viruses, fungi, and parasites. This resistance is achieved through a complex network of cells, tissues, and organs collectively known as the immune system. The immune response can be broadly categorized into innate and adaptive immunity. Adaptive immunity, characterized by specificity and memory, is further divided into humoral (antibody-mediated) and cell-mediated immunity. Cell-mediated immunity is particularly important for combating intracellular pathogens and cancer cells, relying on the direct action of immune cells rather than antibodies.
Defining Immunity
Immunity can be defined as the body’s ability to defend itself against substances that cause disease. This defense system recognizes and neutralizes foreign objects, such as bacteria, viruses, fungi, and parasites. It distinguishes between ‘self’ and ‘non-self’ antigens.
Cell-Mediated Immune Response in Vertebrates
Cell-mediated immunity is an adaptive immune response that doesn’t involve antibodies. It relies on T lymphocytes to directly kill infected cells or activate other immune cells to enhance their function. The process can be broken down into several key steps:
1. Antigen Presentation
Antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells, engulf pathogens and process their antigens. These antigens are then presented on the cell surface bound to Major Histocompatibility Complex (MHC) molecules. MHC class I presents antigens from within the cell (e.g., viral proteins) to cytotoxic T cells (CD8+ T cells), while MHC class II presents antigens from outside the cell (e.g., bacterial proteins) to helper T cells (CD4+ T cells).
2. T-Cell Activation
T cells recognize antigens presented on MHC molecules via their T-cell receptors (TCRs). This interaction, along with co-stimulatory signals, activates the T cell. CD4+ helper T cells are activated by antigen presented on MHC class II, while CD8+ cytotoxic T cells are activated by antigen presented on MHC class I.
3. Clonal Expansion
Once activated, T cells undergo clonal expansion, rapidly dividing to create a large population of identical T cells specific to the antigen. This amplification ensures a sufficient number of effector cells to combat the infection.
4. Effector Mechanisms
- Cytotoxic T Cells (CD8+): These cells directly kill infected cells by releasing cytotoxic granules containing perforin and granzymes. Perforin creates pores in the target cell membrane, allowing granzymes to enter and induce apoptosis (programmed cell death).
- Helper T Cells (CD4+): These cells don’t directly kill infected cells but play a crucial role in coordinating the immune response. They release cytokines that activate macrophages, B cells, and other T cells, enhancing their ability to fight infection. Different subtypes of helper T cells (Th1, Th2, Th17) produce different cytokines and mediate different aspects of the immune response.
5. Memory Cell Formation
Following the resolution of the infection, some activated T cells differentiate into memory T cells. These long-lived cells remain in the body and provide a rapid and enhanced response upon subsequent exposure to the same antigen, conferring immunological memory.
Diagram illustrating the cell-mediated immune response. (Source: Wikimedia Commons)
Comparison of CD4+ and CD8+ T Cells
| Feature | CD4+ T Cells (Helper T Cells) | CD8+ T Cells (Cytotoxic T Cells) |
|---|---|---|
| MHC Restriction | MHC Class II | MHC Class I |
| Function | Activate other immune cells (B cells, macrophages, CD8+ T cells) | Kill infected cells, cancer cells |
| Cytokine Production | Produce various cytokines to regulate immune response | Produce cytotoxic granules (perforin, granzymes) |
Conclusion
Cell-mediated immunity is a vital component of the adaptive immune system, providing protection against intracellular pathogens and cancer. The coordinated action of T lymphocytes, antigen-presenting cells, and cytokines ensures an effective and targeted immune response. Understanding the intricacies of this process is crucial for developing strategies to combat infectious diseases, autoimmune disorders, and cancer, and for improving vaccine efficacy. Further research into T cell biology continues to reveal new insights into the complexities of immune regulation.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.