AQA A-Level Biology: Immune System: Antigens, Antibodies, and Immunity — mark scheme explained

The question

Explain how T H cells stimulate cytotoxic T cells (T C cells), B cells, and phagocytes. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Identify the role of T H cells in the immune response.

• S2

  T H cells are activated by antigens presented by antigen-presenting cells (APCs).

• S3

  Step 2: Describe how T H cells stimulate cytotoxic T cells (T C cells).

• S4

  Activated T H cells release cytokines that activate cytotoxic T cells, which then directly kill infected or abnormal cells.

• S5

  Step 3: Explain the stimulation of B cells by T H cells.

• S6

  T H cells also release cytokines that stimulate B cells to differentiate into plasma cells and produce antibodies.

• S7

  Step 4: Describe how T H cells enhance phagocytosis.

• S8

  Cytokines released by T H cells activate phagocytes, such as macrophages, which then engulf and destroy pathogens.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Identify the role of T H cells in the immune response.

2. S2

   T H cells are activated by antigens presented by antigen-presenting cells (APCs).

3. S3

   Step 2: Describe how T H cells stimulate cytotoxic T cells (T C cells).

4. S4

   Activated T H cells release cytokines that activate cytotoxic T cells, which then directly kill infected or abnormal cells.

5. S5

   Step 3: Explain the stimulation of B cells by T H cells.

6. S6

   T H cells also release cytokines that stimulate B cells to differentiate into plasma cells and produce antibodies.

7. S7

   Step 4: Describe how T H cells enhance phagocytosis.

8. S8

   Cytokines released by T H cells activate phagocytes, such as macrophages, which then engulf and destroy pathogens.

9. Final answer: T H cells stimulate cytotoxic T cells (T C cells), B cells, and phagocytes through the release of cytokines, enhancing the immune response against pathogens.

Common mistakes

• Confusing antigens with antibodies. — Remember that antigens are substances that trigger an immune response, while antibodies are proteins produced by B cells to bind to and neutralize antigens.
• Misunderstanding the structure of antibodies. — Practice visualizing the structure of antibodies, focusing on the tips (variable regions) that bind to antigens and the stem (constant regions) that interacts with other immune components.
• Confusing active and passive immunity. — Remember that active immunity is acquired through natural infection or vaccination, where the body produces its own antibodies, while passive immunity is acquired through the transfer of pre-formed antibodies from another source.
• Failing to explain the role of T H cells in stimulating other immune cells. — Practice explaining the role of T H cells in detail, focusing on the release of cytokines that stimulate other immune cells to enhance the response.
• Incorrectly describing the process of clonal selection in B lymphocytes. — Practice describing the activation, proliferation, and differentiation of B cells into plasma cells and memory cells, emphasizing the production of antibodies.
• Failing to recognize the limitations of antibiotics against viruses. — Remember that antibiotics target bacterial structures and processes, which do not exist in viruses. Viruses replicate inside host cells using the host's machinery, making them resistant to antibiotics.

Where the marks go

• Full worked solution (all marking points)4 marks

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