Cell-Mediated Immunity (CMI)

Cell-Mediated Immunity (CMI): An Overview

Definition

Cell-mediated immunity (CMI) is an immune response that primarily involves the activation of T cells and does not rely on antibodies. It plays a crucial role in defending the body against intracellular pathogens, such as viruses and certain bacteria, as well as in tumor surveillance and transplant rejection.

Key Components of CMI

• T Cells:

• Cytotoxic T Lymphocytes (CTLs): These cells are responsible for directly killing infected or cancerous cells. They recognize specific antigens presented by Major Histocompatibility Complex (MHC) molecules on the surface of infected cells.
• Helper T Cells (Th): These cells assist in the activation of CTLs and other immune cells by releasing cytokines. There are different subsets, including Th1 and Th2 cells, each playing distinct roles in the immune response.

• Antigen-Presenting Cells (APCs):

• Dendritic Cells: These are key players in initiating CMI by capturing antigens and presenting them to T cells. They provide the necessary signals for T cell activation.
• Macrophages: They not only present antigens but also secrete cytokines that further enhance the immune response.

• Cytokines:

• Cytokines are signaling molecules released by immune cells that regulate the intensity and duration of immune responses. They help coordinate the activities of various immune cells involved in CMI.

Mechanism of Action

• Activation Phase:

• When APCs present antigens to naive T cells, it triggers their activation. This process requires two signals: recognition of the antigen-MHC complex by the T cell receptor (TCR) and additional costimulatory signals from APCs.

• Clonal Expansion:

• Once activated, T cells undergo clonal expansion, producing many identical copies that can effectively respond to the same antigen.

• Effector Phase:

• Activated CTLs migrate to sites of infection or tumor growth where they recognize and kill target cells displaying foreign antigens. Helper T cells release cytokines that enhance the activity of other immune cells, including macrophages and B cells.

Functions of CMI

• Destruction of Infected Cells: CMI is particularly effective against virus-infected cells such as COVID-19 and HIVS, intracellular bacteria, and cancer cells.
• Activation of Macrophages and NK Cells: Cytokines released during CMI enhance the ability of macrophages to phagocytize pathogens and activate natural killer (NK) cells to destroy infected or malignant cells.
• Role in Transplant Rejection: CMI is a key factor in the rejection of transplanted tissues due to recognition of foreign antigens.

Clinical Relevance

• Infectious Diseases: Understanding CMI is crucial for developing vaccines and therapies against viral infections.
• Cancer Immunotherapy: Enhancing CMI through checkpoint inhibitors or CAR-T and NK cell therapies has shown promise in treating various blood cancer and solid organ tumor.
• Autoimmune Disorders: Dysregulation of CMI can lead to autoimmune diseases where the immune system attacks healthy tissues.

Conclusion

Cell-mediated immunity is a vital aspect of the adaptive immune response, providing protection against intracellular pathogens and cancerous cells. Its mechanisms involve complex interactions between various immune cells, highlighting its importance in maintaining health and combating diseases.

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References

1. Biology LibreTexts. (2023). “Cell-Mediated Immunity.” Retrieved from LibreTexts.
2. Study.com. (2023). “Cell Mediated Immune Response.” Retrieved from Study.com.
3. Wikipedia contributors. (2023). “Cell-mediated immunity.” In Wikipedia, The Free Encyclopedia. Retrieved from Wikipedia.
4. PubMed Central. (2020). “Evaluation of Cell-Mediated Immune Function.” Retrieved from PMC.
5. NCBI. (2021). “Cell-Mediated Immunity for SARS-CoV-2 Infection.” Retrieved from NCBI.