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The specific response: selected, cloned, remembered questions
Antigens and the distinction between self and non-self; the cellular response with T-helper and T-killer cells, clonal selection and clonal expansion; the humoral response with B cells, plasma cells and memory cells; primary against secondary responses; and HIV as the infection that attacks the co-ordinating cell itself.
5 original questions · 19 marks · the specific response: selected, cloned, remembered notes · Pathogens, disease and immunity
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Describe the sequence of events from a pathogen entering the body to antibody appearing in the blood plasma.
Mark scheme
- B1 the pathogen is engulfed by a phagocyte, which hydrolyses it and presents its antigens on its own cell-surface membrane
- B1 a T-helper cell with a complementary receptor binds to the presented antigen and is activated, which is clonal selection, and divides by mitosis
- B1 the same antigen binds to the surface antibody of the B cell that fits it, and that B cell takes the antigen in, processes it and presents it
- B1 an activated T-helper cell binds to the presenting B cell and stimulates it to divide by mitosis, which is clonal expansion
- B1 the clone differentiates into plasma cells, which secrete antibody, and memory cells, which secrete nothing
Explain why detectable antibody appears about eight days after a first exposure to an antigen but only about two days after a second exposure to the same antigen.
Mark scheme
- B1 on the first exposure the antigen must first be presented by a phagocyte, and the few lymphocytes with a complementary receptor must be selected out of millions
- B1 those selected cells must then divide repeatedly by mitosis before enough exist to matter, and only then do plasma cells appear and begin secreting
- B1 on the second exposure, memory cells specific to that antigen are already circulating in thousands rather than ones, so selection has already happened
- B1 the memory cells divide rapidly straight into plasma cells, so antibody is secreted in quantity within about two days
Explain why an untreated HIV infection eventually leaves a person unable to deal with infections that an intact immune system would handle easily.
Mark scheme
- B1 HIV infects T-helper cells specifically, using the CD4 protein on their surface as its way in, and replicates inside them
- B1 the infected cells are destroyed as new virus particles are released, so over years the number of T-helper cells falls
- B1 B cells are no longer properly activated, so few plasma cells are produced and little antibody is secreted
- B1 T-killer cells are no longer activated, so infected body cells are not destroyed; the infections that follow are what constitute AIDS
A virus is replicating inside the cells lining a person's airways. Suggest how T-killer cells limit the infection, and suggest why antibody alone would not be enough.
State what is meant by clonal selection and state what is meant by clonal expansion.
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