A-Level Biology October/November 2024 Q5(a)(ii): Describe the roles of cells U4 and V4 in a primary immune response.

1 answer

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  Role of Cell U4 (T-helper cell)

  Cell U4 is a T-helper lymphocyte. Its primary role is to coordinate the immune response by releasing chemical signalling molecules called cytokines (or interleukins). These cytokines have several effects, including stimulating B-lymphocytes that have been activated by an antigen. This stimulation causes the B-lymphocytes to divide rapidly by mitosis (clonal expansion) and then differentiate into plasma cells, which produce antibodies, and memory cells.

  Role of Cell V4 (T-killer cell)

  Cell V4 is a T-killer (or cytotoxic T) lymphocyte. Its role is to destroy the body's own cells that have become infected with a pathogen. It does this by binding to the infected cell and releasing cytotoxic chemicals. For example, it releases a protein called perforin, which creates pores in the cell surface membrane of the infected cell. This allows other molecules, like granzymes, to enter and triggers cell death by lysis or apoptosis.

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  How the marks are awarded

  • B1 — Correctly stating that cell U4 (T-helper cell) secretes or releases cytokines/interleukins, as mentioned in the first sentence of the answer.
  • B1 — Describing how these cytokines stimulate B-lymphocytes to divide and develop into plasma cells, as explained in the second sentence for cell U4.
  • B1 — Stating that cell V4 (T-killer cell) releases chemicals specifically to kill or break down infected host cells, as described in the first two sentences for cell V4.
  • B1 — Naming a specific chemical released by V4 (perforin) and describing its action (creates pores in the cell surface membrane), covering two linked marking points from the scheme.

  Common mistakes

  • Confusing the roles of T-helper (U4) and T-killer (V4) cells, for example, stating that T-helper cells kill infected cells.
  • Incorrectly stating that T-cells produce antibodies; this is the function of plasma cells, which differentiate from B-cells.
  • Giving a vague description of the T-killer cell's role, such as 'it kills pathogens', instead of specifying that it kills the body's own infected cells.
  • Failing to name a specific cytotoxic chemical (like perforin or granzymes) or describing its mechanism of action incorrectly.

  Examiner tip: Create a detailed flowchart or summary table for the cell-mediated and humoral immune responses to clearly distinguish the specific roles and interactions of each type of lymphocyte.

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