--- title: "Adaptive Immunity: Antigen Presentation and T-Cell Selection" description: "Cells display fragments of the proteins inside them on surface molecules, and T cells inspect those displays. Because the useful receptors are selected during development, the system reacts to what is" canonical: https://lightmysky.com/learn/science/adaptive-immunity-antigen-presentation-and-t-cell-selection-mt_8hTGqrnqGY source: https://lightmysky.com/learn/science/adaptive-immunity-antigen-presentation-and-t-cell-selection-mt_8hTGqrnqGY.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Adaptive Immunity: Antigen Presentation and T-Cell Selection Cells display fragments of the proteins inside them on surface molecules, and T cells inspect those displays. Because the useful receptors are selected during development, the system reacts to what is foreign and normally ignores what is not. Subject: Science · Area: The Human Body · Ages 21 to 22 Page: https://lightmysky.com/learn/science/adaptive-immunity-antigen-presentation-and-t-cell-selection-mt_8hTGqrnqGY ## Ready when they can - Distinguishes display of internal proteins from display of engulfed material by which T cell responds. - Explains how selection during development produces a repertoire that is useful and tolerant. - Says why a virus-infected cell can be killed while a healthy neighbour is not. ## Lesson: Show your insides: how T cells spot infection Your cells display fragments of the proteins inside them on surface molecules called MHC. MHC class I sits on all nucleated cells, and mature red blood cells are the only ones lacking it. It shows bits of whatever the cell is making, self pieces in health and viral pieces in infection. Killer T cells, the CD8 type, inspect this display. A second display handles engulfed material. Only three cell types carry MHC class II: macrophages, dendritic cells, and B cells. They swallow outsiders, chop them up, and show the pieces to helper T cells, the CD4 type. So internal proteins summon killers through class I, while eaten material summons helpers through class II. **Example.** Picture two neighbours during a virus outbreak. The healthy cell shows only self fragments on class I, and the patrolling killer T cell moves on. The infected cell shows viral fragments on the same class I molecules, and the killer destroys it while the healthy neighbour stands untouched. **Tip.** Useful and tolerant T cells are built by selection in the thymus. First the cell must build a working receptor, then it must bind MHC at all, and finally it must not bind self. Failures die by apoptosis, and this central tolerance is backed by peripheral controls that quiet escapees. **Recap.** Class I shows insides to killers, class II shows eaten material to helpers, and thymus selection keeps the repertoire useful and tolerant. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Innate Immunity: Pattern Recognition, Complement and Inflammation](https://lightmysky.com/learn/science/innate-immunity-pattern-recognition-complement-and-inflammation-mt_3TcgoOGI0L) ## Opens up - [Antibody Diversity, Memory and What Tolerance Failure Costs](https://lightmysky.com/learn/science/antibody-diversity-memory-and-what-tolerance-failure-costs-mt_5YXKhiOqoA) - [Checkpoint Blockade: Taking a Brake Off a T Cell](https://lightmysky.com/learn/science/checkpoint-blockade-taking-a-brake-off-a-t-cell-mt_f9P4LlaBGb)