Our bodies use antibodies to check fluids between cells, some pathogens are clever enough to hide inside cells themselves. This is where Cell-Mediated Immunity (CMI) steps in. Unlike humoral immunity (which uses B-cells and antibodies), CMI does not rely on circulating proteins. Instead, it involves activation of specialized cells that hunt down and destroy infected or abnormal host cells.
How Cell-Mediated Immunity Works
The process is a highly coordinated led by T-lymphocytes (T-cells). Since T-cells cannot see pathogens floating freely, they rely on a system of cellular flags called Major Histocompatibility Complex (MHC) molecules.
1. Antigen Recognition and Presentation
When a virus infects a cell or a macrophage swallows a bacterium, pieces of the pathogen (antigens) are displayed on cell surface via MHC molecules.
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MHC Class I: Found on almost all nucleated cells; it signals like “I am infected, kill me.”
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MHC Class II: Found on specialized immune cells (dendritic cells); it signals like “I found a threat, let’s coordinate a response.”
2. Key Players
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Helper T-cells: Once they recognize an antigen on MHC Class II molecule, they release cytokines—chemical alarms that bring other immune cells to fight.
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Cytotoxic T-cells: They recognize antigens on MHC Class I molecules. Once activated, they bind to target cell and release perforins (which poke holes in membrane) and granzymes (which trigger programmed cell death, or apoptosis).
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Natural Killer (NK) Cells: These act like a rapid team, attacking cells that show signs of stress or lack proper self markers, even before T-cells are fully mobilized.
When is CMI Most Important?
Cell-mediated immunity is our primary defense against threats that antibodies can’t reach:
| Threat Type | How CMI Handles It |
| Intracellular Pathogens | Destroys cells harboring viruses, bacteria (like M. tuberculosis), and fungi. |
| Cancer Cells | Identifies and eliminates mutated host cells before they can form tumors. |
| Transplanted Tissue | Unfortunately, CMI is main reason for organ transplant rejection, as it views new organ as foreign. |
Memory
After infection is cleared, most activated T-cells die off. However, a small population remains as Memory T-cells. These cells remember specific antigen for years. If same pathogen tries to invade again, these memory cells trigger a response so rapid and aggressive that you often don’t even feel symptoms second time around.
While CMI is important for health, an overactive cell-mediated response can lead to autoimmune diseases, where the body’s begin attacking healthy, normal tissue.