The function of cell-mediated immunity (CMI) is to provide an immune response against antigens that does not rely on antibodies.
Cell-mediated immunity is a crucial part of the immune system's defense strategy. Unlike humoral immunity, which uses antibodies circulating in the blood and lymph, CMI involves the direct action of specialized immune cells.
Key Components and Their Roles in Cell-Mediated Immunity
Based on the provided information, the function of CMI is carried out through the coordinated action of several cell types and molecules in response to an antigen:
- Activation of Macrophages and NK-cells: CMI involves activating these innate immune cells.
- Macrophages are phagocytic cells that engulf and digest pathogens or infected cells. Activation enhances their killing ability.
- Natural Killer (NK) cells are lymphocytes that can kill target cells (like tumor cells or virus-infected cells) without prior sensitization.
- Production of Antigen-Specific Cytotoxic T-Lymphocytes: A hallmark of CMI is the generation of cytotoxic T cells (also known as killer T cells). These cells are highly specific to a particular antigen. Their primary function is to identify and directly kill cells that are displaying that antigen, such as infected cells or abnormal cells.
- Release of Various Cytokines: In response to the antigen, various immune cells involved in CMI (like T cells and macrophages) release signaling molecules called cytokines.
- Cytokines have diverse functions, including communicating with other immune cells, promoting inflammation, and enhancing the killing activity of cells like macrophages and cytotoxic T lymphocytes.
How CMI Works
In essence, when an antigen is detected (e.g., from a virus-infected cell or an intracellular bacterium), the cell-mediated immune response is triggered. This response mobilizes cells that can directly attack and eliminate the threat or assist other cells in doing so, without involving antibodies.
Summary of CMI Actions:
- Directly killing infected or target cells (primarily by cytotoxic T lymphocytes).
- Activating other immune cells (like macrophages and NK cells) to enhance their ability to destroy pathogens or infected cells.
- Using chemical signals (cytokines) to coordinate the immune response and amplify its effects.
This cellular-level defense is particularly effective against pathogens that reside inside cells (like viruses and some bacteria), transplanted tissues, and cancer cells, which are often less accessible to antibodies.
