Overview
This lecture summarizes the mechanisms of cell-mediated immunity provided by natural killer (NK) cells and CD8+ cytotoxic T cells, highlighting their activation and roles in targeting infected or cancerous cells.
Types of T Cells and Immunity
- The adaptive immune system includes B cells and T cells (helper T cells with CD4, cytotoxic T cells with CD8).
- Helper T cells assist other immune cells; cytotoxic T cells directly kill infected or abnormal cells.
- NK cells and cytotoxic T cells are central to cell-mediated immunity, which depends on direct cell interactions.
Activation of Cytotoxic T Cells (CD8+)
- Naive T cells become activated after encountering an antigen in lymph nodes.
- Activation requires two signals: antigen presentation (on MHC I via antigen-presenting cells) and costimulation (CD28 on T cell binds B7 on APC).
- Cross-presentation allows dendritic cells or macrophages to present antigens from pathogens or tumors to CD8+ T cells.
- Upon activation, cytotoxic T cells upregulate high-affinity IL-2 receptors, respond to IL-2, and undergo clonal expansion.
Effector Function and Target Recognition
- Activated cytotoxic T cells require only antigen-MHC I recognition for killing, not costimulatory signals.
- Cytotoxic T cells use adhesion molecules (LFA-1 and ICAM) to scan and bind target cells.
- Targeted killing occurs if the correct antigen is recognized on MHC I; otherwise, the T cell detaches and moves on.
Natural Killer (NK) Cell Functions
- NK cells are innate lymphocytes involved in early defense against viral infections and cancer.
- NK cells kill infected or tumor cells using perforin and granzymes, inducing apoptosis.
- NK cells do not require antigen presentation via MHC for activation.
Summary and Importance
- Both NK cells and CD8+ T cells are essential for eliminating infected or cancerous cells and preventing disease spread.
- CD8+ T cells also secrete cytokines to further stimulate the immune response.
Key Terms & Definitions
- Cytotoxic T cell (CD8+) — Adaptive immune cell that kills infected or abnormal cells by recognizing antigens on MHC I molecules.
- Natural killer (NK) cell — Innate immune lymphocyte that destroys infected or cancerous cells without prior sensitization.
- Cell-mediated immunity — Immune defense involving direct actions of immune cells rather than antibodies.
- MHC I — Molecule on cell surfaces presenting intracellular antigens to CD8+ T cells.
- Antigen presenting cell (APC) — Cell (e.g., macrophage, dendritic cell) that displays antigens to T cells to initiate their activation.
- Costimulation — Second signal required for full T cell activation, involving CD28 on T cells and B7 on APCs.
Action Items / Next Steps
- Review mechanisms of antigen presentation and T cell activation.
- Study differences between innate (NK cells) and adaptive (CD8+ T cells) cell-mediated immunity.
- Complete assigned flashcards and USMLE-style practice questions on this topic.