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.
What a learner can do afterwards
- 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.
1 · Read
Your cells display fragments of the proteins inside them on surface molecules called MHC. MHC class I sits on all nucleated cells, which leaves mature red blood cells without 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. Three main display cells 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.
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.
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.
Class I shows insides to killers, class II shows eaten material to helpers, and thymus selection keeps the repertoire useful and tolerant.
The rewritten readings were written for this dataset.
More to read
Recognising invaders and remembering them
Adaptive immunity develops through exposure. Lymphocytes have different receptors; those that recognise an invading antigen can activate and multiply.
Antibodies provide humoral immunity, while T cells provide cellular immunity. Both arms of adaptive immunity develop memory, allowing a later response to the same antigen to develop more quickly and strongly.
Displaying antigens to T cells
Most T cells recognise antigen fragments carried by MHC molecules. Class I usually presents proteins from inside cells; class II usually presents material taken up from outside.
Professional antigen-presenting cells use class II to activate helper T cells. Dendritic cells collect antigens in tissues and carry them to lymph nodes.
Selecting T cells and controlling their targets
During maturation, T cells must recognise self MHC to survive. Many that react strongly to self antigens are removed. This selection supports useful responses and limits self attack, but tolerance also needs other controls.
Cytotoxic T cells destroy infected cells presenting foreign fragments on MHC I. Helper T cells coordinate other immune cells. Together, selection and tolerance mechanisms help protect healthy tissue.
From antigen recognition to immune memory
Antigen presentation leads to lymphocyte activation, an attack and lasting memory. Failures early in this sequence can impair later responses.
Random receptor generation creates variety; selection limits reactions against self. Together these processes allow responses to unfamiliar threats. Vaccination builds protective memory without requiring the illness caused by natural infection.
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2 · Watch
En anglaisTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.