explain the role of memory cells in the secondary immune response and in long-term immunity

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – The Immune System: Memory Cells

The Immune System – Memory Cells and the Secondary Immune Response

Learning Objective

Explain the role of memory cells in the secondary immune response and in long‑term immunity.

Key Concepts

  • Memory B cells and memory T cells are generated during the primary immune response.

  • These cells persist for years, sometimes for the lifetime of the individual.

  • On re‑exposure to the same antigen, memory cells mount a rapid and robust secondary response.

  • The secondary response underlies the principle of vaccination and long‑term protective immunity.

Formation of Memory Cells

During the primary response, activated naïve B and T lymphocytes proliferate and differentiate into:

  1. Effector cells – short‑lived cells that eliminate the current infection.

  2. Memory cells – long‑lived cells that retain antigen‑specific receptors.

Memory cells are maintained in peripheral lymphoid tissues and circulate in the bloodstream, ready to respond quickly upon re‑encounter with their specific antigen.

Comparison: Primary vs. Secondary Immune Response

FeaturePrimary ResponseSecondary Response
Lag time before detectable antibodies\overline{5}–7 days\overline{1}–3 days
Antibody titre (peak concentration)Low to moderate (≈10⁻⁸ M)High (≈10⁻⁶ M or greater)
Antibody class predominanceIgM → IgGPredominantly IgG (high‑affinity)
Affinity of antibodiesLow to moderate affinityHigh affinity due to somatic hypermutation
Cellular participantsNaïve B and T cells activatedMemory B and T cells re‑activated
Duration of protectionShort‑lived (weeks)Long‑lived (months to years)

Mechanisms Underlying the Rapid Secondary Response

  • Higher precursor frequency: Memory cells are more numerous than naïve cells for a given antigen.

  • Lower activation threshold: Memory lymphocytes require less co‑stimulatory signalling.

  • Pre‑formed antibodies: Circulating low‑level IgG from the primary response can neutralise pathogen immediately.

  • Enhanced antigen presentation: Memory B cells can act as efficient antigen‑presenting cells to T helper cells.

Long‑Term Immunity

Long‑term immunity is maintained by the persistence of memory cells and, for humoral immunity, by long‑lived plasma cells that continuously secrete antibodies. The half‑life of IgG in serum is about 21 days, but continual production by plasma cells sustains protective levels.

Mathematically, the steady‑state concentration of antibody \$[Ab]\$ can be expressed as:

\$[Ab] = \frac{P}{k_{\text{deg}}}\$

where \$P\$ is the rate of antibody production by plasma cells and \$k_{\text{deg}}\$ is the degradation constant.

Clinical Relevance

  • Vaccination: Introduces a harmless form of antigen to generate memory cells without causing disease.

  • Booster doses: Re‑expose the immune system to increase memory cell numbers and antibody titres.

  • Immunological memory loss: Can occur with ageing (immunosenescence) or immunosuppressive therapy, reducing vaccine efficacy.

Suggested diagram: Flowchart showing the generation of memory B cells during the primary response and their rapid activation during the secondary response.

Summary

Memory cells are the cornerstone of the secondary immune response and long‑term immunity. Their ability to recognise previously encountered antigens, respond more quickly, and produce high‑affinity antibodies provides the biological basis for vaccination, booster programmes, and lasting protection against many infectious diseases.