Helper T Cells ( / CD4+): These cells act as the coordinators of the immune system. Upon activation, they secrete cytokines that stimulate B cell antibody production, enhance the killing power of macrophages, and promote the growth of cytotoxic T cells.
Cytotoxic T Cells ( / CD8+): These are the 'effector' killers that directly eliminate infected or abnormal host cells. They release perforins, which create holes in the target cell membrane, and granzymes, which enter the holes to trigger apoptosis (programmed cell death).
Regulatory T Cells (): These cells serve to dampen the immune response once a pathogen is cleared. They are essential for preventing overactive immune responses that could damage healthy host tissues.
| Feature | Helper T Cells () | Cytotoxic T Cells () |
|---|---|---|
| Surface Marker | CD4 | CD8 |
| MHC Interaction | MHC Class II | MHC Class I |
| Primary Function | Cytokine secretion / Coordination | Direct cell lysis / Apoptosis |
| Target Cells | APCs (Dendritic, Macrophages) | Any nucleated cell (Infected/Cancer) |
The Rule of 8: A helpful mnemonic for remembering MHC restriction is the 'Rule of 8'. and . If the product of the cell type and the MHC class equals 8, the pairing is correct.
Check for Co-stimulation: In exam scenarios involving T cell activation, always look for the presence of a second signal. If only the TCR-MHC interaction is described, the result is likely anergy, not activation.
Distinguish Killing Mechanisms: Ensure you can differentiate between the humoral response (antibodies) and the cell-mediated response (T cells). T cells do not produce antibodies; they either help other cells or kill targets directly.
Common Mistake: Students often forget that T cells cannot see 'naked' antigens. If a question asks if a T cell will bind to a free virus in the bloodstream, the answer is always 'No'.
