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Primary and Secondary Immune Response

Laura Armstrong & Joe Wolfensohn

Teachers

Laura Armstrong Joe Wolfensohn

Contents

Topic Explainer Videos

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Primary Immune System

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Secondary Immune Response

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Comparison Table: Primary Response vs. Secondary Response

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Recall Questions

This topic requires prior knowledge of the humoral response and the formation of memory cells. You can test your knowledge on these below.

What are T-cells?

  • T-cells are a type of white blood cell (lymphocyte) involved in the cellular response.
  • Types of T-cells include Helper T-cells, which activate B-cells, Cytotoxic T-cells (Killer T-cells), which kill infected or cancerous cells and Memory T-cells which provide long-term immunity.

What are B-cells?

  • B-cells are a type of white blood cell that is responsible for humoral immunity.
  • When activated, B-cells differentiate into plasma cells (which produce antibodies) and Memory B-cells (which provide long-term immunity).

What are memory cells?

  • Memory cells are long-lived immune cells that remain in the body after an infection has been cleared.
  • They allow for a faster and stronger secondary immune response if the same pathogen infects the body again.
  • Both Memory T-cells and Memory B-cells exist to provide long-term immunity.

Topic Explainer Videos

Check out this @LauraDoesBiology video that explains primary and secondary immune response or read the full notes below. Once you've gone through the whole note, try out the practice questions!

Primary Immune System

  • Occurs when the immune system encounters a pathogen for the first time.
  • It is slow because:

    • The immune system must identify the antigen.

    • Specific B-cells must be activated and clone themselves (clonal selection and clonal expansion must take place).

    • Plasma cells produce antibodies, which take time to reach a high concentration.

  • Symptoms of infection occur as the pathogen spreads before it is eliminated.
  • After the infection is cleared, memory cells (both B and T memory cells) are produced and remain in the body.

Secondary Immune Response

  • Happens when the same pathogen, with the same antigen, infects the body again.
  • It is much faster and stronger because:

    • Memory B-cells quickly differentiate into plasma cells and produce antibodies.

    • Memory T-cells rapidly activate cytotoxic T-cells to destroy infected cells.

    • More antibodies are produced, and are produced more rapidly. They also remain in the bloodstream for longer.

  • The pathogen is eliminated before symptoms appear, meaning the person may not even realise they were infected.

This graph shows the difference in antibody concentration following a primary and a secondary immune response. Following the secondary response:

  • There is very little delay or lag time before antibody production begins.
  • The antibody concentration increases more quickly.
  • The antibody concentration rises much higher.
  • The antibody concentration remains higher for longer.

This is the result of Memory cells being present following the primary immune response.

Comparison Table: Primary Response vs. Secondary Response

Feature Primary Response  Secondary Response
Speed Slow. Fast.
Antibody production Takes time (few days). Rapid (few hours).
Antibody concentration Lower. Much higher.
Symptoms Present. Usually absent.
Memory cells involved? No, but they are produced. Already Present from primary response.

Key Terms

  • Antigen: A protein on the surface of a pathogen that triggers an immune response.
  • Antibody: A protein produced by B-cells (plasma cells) that binds to antigens.
  • Memory Cells: Long-lived cells that allow a faster secondary response.
  • Vaccination: Artificial exposure to antigens to stimulate memory cell formation without causing disease.
No answer provided.

Exam Tips

"Describe" vs. "Explain"

If a question asks you to describe the difference between the two responses, focus on observations (e.g., speed, antibody concentration).

If it asks you to explain, include the biological mechanisms (e.g., the role of memory cells in the secondary response and that there is no need for clonal selection of the correct B-cell).

No answer provided.

Explain why the secondary immune response is faster and produces more antibodies than the primary response. (4 marks)

  • Memory B and T-cells remain in the body after the primary infection.
  • Upon re-exposure, memory B-cells differentiate rapidly into plasma cells.
  • Plasma cells produce antibodies more quickly and in higher concentrations.
  • The pathogen is destroyed before symptoms develop.

Check your understanding

Use this video to check your understanding of the primary and secondary immune response!

The humoral response Antibodies