Loading...
An immune response links recognition to the activation of appropriate cells. Explain how antigen presentation connects an early phagocyte response with a specific lymphocyte response.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Define an antigen and distinguish it from an antibody
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: An antigen is best described here as:
A molecular feature recognised by a specific immune response. Keep recognition targets distinct from immune effectors.
After watching 2: Lysosomes contribute to phagocytosis by:
Providing hydrolytic enzymes for digestion. Engulfment is followed by controlled intracellular digestion.
What you'll be able to do
Cells and pathogens carry molecules that can be recognised by immune-system receptors. An antigen is a molecule or molecular feature recognised as part of a specific immune response; it is often a protein. Different cell-surface molecules help distinguish pathogens, transplanted cells and abnormal body cells.
An antigen is not the same as an antibody, and it need not be a complete living pathogen. Variation in pathogen antigens can mean that a response effective against one variant is less effective against another. Recognition is specific because receptor binding depends on complementary molecular features.
A phagocyte recognises and engulfs a pathogen, enclosing it in a vesicle. Lysosomes fuse with the vesicle and hydrolytic enzymes digest the ingested material. Enzymes such as lysozyme contribute to breaking down appropriate microbial structures; the pathogen is not simply absorbed intact into the nucleus.
Some cells display processed antigen at their surface and act as antigen-presenting cells. Presentation allows specific T lymphocytes to recognise that antigen in the appropriate cellular context. The ingested whole pathogen and the displayed antigen fragment are not identical objects, so describe the processing step clearly.
A helper T cell with a complementary receptor can be activated following appropriate antigen recognition and presentation. It proliferates by mitosis to form a clone and releases signals that stimulate other immune cells, including B cells, cytotoxic T cells and phagocytes. This connects different arms of the response.
Cytotoxic T cells recognise suitable antigen on infected or abnormal body cells and can trigger their destruction. Their main target is the host cell displaying the antigen, not every free antibody in the blood. The cellular response therefore complements antibody-mediated defence rather than replacing it. Avoid describing all lymphocytes as cells that engulf pathogens.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.