(g) the primary and secondary immune responses

The idea

The primary immune response follows first exposure to an antigen. It is slow because the few lymphocytes with a matching receptor must be selected, then divide and differentiate. Antibody concentration rises gradually and may take days, and memory cells are produced. On later exposure to the same antigen, memory B and T cells respond quickly, so the secondary response is faster, larger and longer-lasting, with a higher antibody concentration. Memory cells also mean the person is often immune to that pathogen. A question may ask you to compare the two responses, interpret a graph of antibody concentration against time, or identify which features belong to the secondary response.

What earns the marks

• The primary response is slow, produces a lower antibody concentration and generates memory cells.
• The secondary response is faster and produces a higher and longer-lasting antibody concentration.
• Memory B and T cells are responsible for the speed and size of the secondary response.
• The secondary response occurs on re-exposure to the same antigen and can prevent symptoms, giving immunity.

Common mistakes and how to put them right

• Saying the secondary response involves a new antigen.

  Put it right: Is that it is triggered by the same antigen on re-exposure.

• Claiming the primary response is faster.

  Put it right: Is that the primary response is slower and the secondary response is faster and larger.

• Ignoring memory cells: correction is that memory cells explain the speed and magnitude of the secondary response.

Exam tips

• When reading a graph, compare the time to peak and the peak height of antibody concentration for each exposure.
• Link each feature of the secondary response back to memory cells rather than to a new set of lymphocytes.
• Check that any option about immunity refers to re-exposure to the same antigen.