A-Level Biology: The Immune System

The immune system is a favourite of A-level examiners because it chains together cell biology, protein specificity and applied medicine. This set — quiz, flashcards, printable written questions and a spoken exam — covers the full immune-response narrative plus the applications exam boards commonly assess.

The cellular questions follow the response in order: phagocyte recognition of pathogens, antigen presentation, and the two adaptive arms — T helper cells coordinating the response through cytokine signalling to B cells and cytotoxic T cells, and the humoral response tested through clonal selection: the identification and expansion of the one B-cell clone whose receptor matches the antigen, producing plasma cells and antibodies.

The memory questions cover the primary-versus-secondary response contrast — why memory B cells make re-exposure faster and stronger — and the immunity taxonomy: active versus passive, tested with concrete examples of each, plus vaccination and the threshold logic of herd immunity.

The applied questions are where A-level papers cash the topic out: antigen variability in HIV and influenza as the reason immunity and vaccines fail against them, HIV's replication mechanism with reverse transcriptase converting viral RNA to DNA, why antibiotics — which target bacterial walls and 70S ribosomes — cannot touch viruses, and the ELISA test, tested on what the enzyme-linked antibody actually does: converting a substrate into a detectable signal.

Each explanation names the cells and mechanisms precisely, at the level mark schemes require, making this set an efficient consolidation of one of the specification's densest sections.

Sequence the immune response from phagocytosis to antibody production
Distinguish the cell-mediated and humoral arms and their coordination by T helper cells
Explain clonal selection and the memory basis of the secondary response
Classify immunity as active or passive with correct examples
Apply the topic to HIV, antibiotics and the ELISA test mechanism

Topic scope follows the immunity content of the DfE A level biology subject content as delivered in current exam-board specifications: phagocytosis, T and B lymphocytes, antibodies, vaccination, HIV and monoclonal antibody applications.

Sample question

Graph of antibody concentration (arbitrary units) against time in days from 0 to 60. After the first exposure at day 0 the concentration rises slowly to a small peak of about 20 at day 12 and falls; after the second exposure, marked by a dashed line at day 30, it rises quickly to a peak of about 100 at day 37 and then falls.

The graph shows the concentration of antibody in the blood after a first exposure to an antigen at day 0 and a second exposure to the same antigen at day 30. Why does the secondary immune response occur much faster and produce higher antibody concentrations than the primary response?

Because the pathogen is weaker during the second exposure.
Because memory B cells can rapidly differentiate into plasma cells upon re-exposure.
Because the body has a higher baseline level of circulating antibodies.
Because T cells are no longer required for the secondary response.

See the answer

Because memory B cells can rapidly differentiate into plasma cells upon re-exposure.

The secondary response is faster because memory B cells, generated during the primary response, are already present and can quickly differentiate into plasma cells.

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