A level Biology B · Edexcel 9BI0 · Topic 6D
The immune response
Key points
- Non-specific defences: skin, mucus and cilia, stomach acid, lysozyme, inflammation and phagocytosis.
- In phagocytosis, a phagocyte engulfs a pathogen into a phagosome, which fuses with a lysosome to digest it; the cell may then act as an antigen-presenting cell.
- T helper cells bind presented antigens and release cytokines; T killer cells destroy infected cells; B cells that match the antigen divide (clonal selection) into plasma cells that secrete antibodies.
- Antibodies are Y-shaped proteins with variable regions complementary to one antigen; they cause agglutination, neutralise toxins and mark pathogens for phagocytosis.
- Memory cells give a faster, larger secondary response.
- Immunity can be active (infection, vaccination) or passive (antibodies across the placenta, in breast milk or by injection); passive immunity is short-lived.
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The body protects itself in layers: barriers keep most pathogens out, non-specific responses deal quickly with anything that gets in, and the specific immune response targets each new antigen and remembers it.
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