CMI Full Form – Understanding Cell-Mediated

Cell-Mediated Immunity (CMI) is a vital defense mechanism of the adaptive immune system that protects the body against infections, cancer, and other abnormal conditions. Unlike humoral immunity, which depends on antibodies produced by B-cells, CMI operates through the direct action of immune cells, particularly T-lymphocytes (T-cells).

CMI is especially crucial for defending against intracellular pathogens—microorganisms that invade and live inside the body’s cells. These include:

• Viruses (e.g., HIV, Influenza, COVID-19)

• Certain bacteria (e.g., Mycobacterium tuberculosis, Salmonella)

• Fungi (e.g., Candida, Histoplasma)

• Parasitic infections (e.g., Leishmania)

• Cancerous or mutated cells

History of Cell-Mediated Immunity

The concept of Cell-Mediated Immunity (CMI) emerged in the late 19th and early 20th centuries when scientists observed immune responses that could not be explained by antibodies alone. In the 1940s, Merrill Chase demonstrated that immune cells, rather than serum antibodies, could transfer immunity, laying the foundation for CMI. Later, in the 1950s and 1960s, immunologists like Peter Medawar and Macfarlane Burnet further elucidated the role of T-cells in immune regulation and transplant rejection. The discovery of major histocompatibility complex (MHC) molecules and cytokines in the 1970s and 1980s significantly advanced our understanding of CMI. Today, CMI is recognized as a crucial component of adaptive immunity, especially in fighting intracellular pathogens and cancer.

Importance of CMI in the Immune System

CMI plays a vital role in:

• Eliminating virus-infected cells and cancerous cells.

• Fighting intracellular bacteria like Mycobacterium tuberculosis.

• Preventing organ transplant rejection.

• Supporting long-term immunity after infections or vaccinations.

How Does Cell-Mediated Immunity Work?

CMI involves a series of immune responses coordinated by T-cells:

1. Antigen Presentation – Antigen-presenting cells (APCs) like macrophages or dendritic cells display foreign antigens to T-helper cells (CD4+ T-cells).

2. T-Cell Activation – CD4+ T-cells stimulate cytotoxic T-cells (CD8+ T-cells) and other immune cells through cytokines.

3. Attack & Destruction – CD8+ T-cells destroy infected or abnormal cells by releasing toxic molecules like perforins and granzymes.

4. Memory T-Cells Formation – These cells provide long-term protection by recognizing previously encountered pathogens.

Key Components of CMI – T-Cells & Cytokines

CMI primarily involves:

• T-Cells:

• Helper T-cells (CD4+): Coordinate immune responses.

• Cytotoxic T-cells (CD8+): Kill infected or abnormal cells.

• Memory T-cells: Provide immunity against future infections.

• Cytokines:

• Chemical messengers like interferons, interleukins, and tumor necrosis factors (TNFs) that regulate immune responses.

Difference Between Cell-Mediated and Humoral Immunity

Induction & Experimental Models of CMI

Assessing CMI is more challenging than measuring antibody responses. Scientists use experimental models such as:

• DTH Tests – Injecting antigens like ovalbumin or tetanus toxoid into animals (e.g., footpad swelling in rats or skin reactions in humans) to measure immune response.

• Flow Cytometry & In Vitro Studies – Used in clinical trials to detect immune activity but can be limited in sensitivity.

• Vaccine Trials – Research on CMI responses in genetically modified mice has demonstrated that cell-mediated immunity is essential for survival against live pathogen exposure.

Role of CMI in Fighting Infections & Diseases

CMI helps combat:

• Viral infections: (HIV, Influenza, COVID-19) by killing infected cells.

• Intracellular bacterial infections: (Tuberculosis, Salmonella) by activating macrophages.

• Fungal infections: (Candida, Histoplasmosis) through T-cell responses.

• Cancer cells: By identifying and destroying mutated or abnormal cells.

• Transplant Rejection: By recognizing and attacking foreign tissue in organ transplants.

Disorders Related to Cell-Mediated Immunity

CMI dysfunction can lead to:

1. Immunodeficiency Disorders:

• HIV/AIDS: The virus attacks CD4+ T-cells, weakening CMI.

• SCID (Severe Combined Immunodeficiency): A genetic disorder where T-cells are absent or non-functional.

2. Autoimmune Diseases:

• Type 1 Diabetes: T-cells attack insulin-producing pancreatic cells.

• Multiple Sclerosis: CMI attacks the protective covering of nerve cells.

3. Hypersensitivity Reactions:

• Contact Dermatitis: CMI overreacts to allergens like poison ivy.

• Graft vs. Host Disease (GVHD): In transplanted organs, immune cells attack host tissues.

Conclusion

Cell-Mediated Immunity (CMI) is a crucial component of the adaptive immune system, playing a vital role in protecting the body against intracellular pathogens, viruses, fungi, and even cancerous cells. Unlike humoral immunity, which relies on antibodies, CMI functions through specialized T-cells and cytokines to directly eliminate infected or abnormal cells. This immune response is essential for long-term immunity, organ transplant compatibility, and the prevention of autoimmune disorders.

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