Summary
Summary AQA A Level Biology- Immune Response Processes
Detailed summary of the relevant immune responses (phagocytosis, cell-mediated response and humoral response) required for the new AQA A Level Biology specification.
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Phagocytosis:1. The phagocyte is attracted to the pathogen’s chemical products, therefore it moves
towards the pathogen along a conc. gradient
2. The receptors on the phagocyte attach to chemicals on the surface of the pathogen.
3. A phagosome is formed when the phagocyte engulfs the pathogen. Lysosomes move
towards the vesicle and fuse to form a phagolysosome.
4. Lysozymes are released into the phagosome and hydrolyse the bacteria.
5. Soluble hydrolysis products are absorbed into the phagocyte’s cytoplasm.
Cell-Mediated Response:
1. Phagocytes take in pathogens.
2. Phagocytes place antigens from pathogens onto its cell-surface membrane.
3. Receptors on a helper T cell fit exactly onto these antigens.
4. This attachment activates the T cells to divide rapidly by mitosis and form a clone of
genetically identical cells.
5. The cloned cells develop into: memory cells that enable a rapid response to future
infections by the same pathogen; stimulate phagocytes to engulf pathogens by
phagocytosis; stimulate B cells to divide and secrete their antibodies; activate
cytotoxic cells.
Humoral Response:
1. The surface antigens of an invading pathogen are taken up by a B cell.
2. The B cell processes the antigens and presents them on its surface.
3. Helper T cells attach to the processed antigens and therefore activates the B cell.
4. The B cell is now activated to divide by mitosis to form a clone of plasma cells.
5. The cloned plasma cells produce and secrete the specific antibody that exactly fits
the antigen on the pathogen’s surface.
6. The antibody attaches to the antigen on the pathogens and destroys them.
7. Some B cells develop into memory cells. These can respond to future infections by
the same pathogen by dividing rapidly and developing into plasma cells that produce
antibodies.
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