Biology · MU Sofia

32. Immunological memory. Primary and secondary immune response.

Covers how antibodies become soluble and how B-cells turn into plasma cells, then explains the contraction of the response and the formation of memory cells. It contrasts the slow primary response with the faster, stronger secondary response, and describes memory B-cells, their class switching and high-affinity receptors, and the T-helper requirement. It closes with the principle of vaccination as priming for a rapid secondary response.

Inside: 1 section · 3 min

Topic 32 of 64 in the MU Sofia syllabus for Biology (Dental Medicine, Year 1). The free AceMed note covers it in 1 section, about 459 words (3 min of reading).

In this note

  1. Introduction

Introduction

Transition to Soluble Antibodies

  • Origin: Initially, antibodies are attached to B-cell membranes as receptors.
  • Function Shift: For active participation in humoral immunity, antibodies need to be released into the bloodstream.
  • Process: This is done through alternative RNA processing, which removes the segment that anchors them to the membrane, allowing antibodies to become soluble without any change to the DNA.

Development into Plasma Cells

  • Transformation: B-cells evolve into plasma cells after encountering antigens and receiving signals from T-helper cells.
  • Changes: These cells stop expressing most membrane-bound immunoglobulin receptors and begin extensive secretion of soluble antibodies.
  • Cellular Adaptation: Their cytoplasm becomes packed with rough endoplasmic reticulum to facilitate antibody production.
  • Final Stage: Plasma cells are end-stage cells; they do not divide and have a short lifespan, focusing solely on antibody secretion.

Immune Response Contraction and Memory

  • Post-Response Reduction: After the antigen is cleared, effector cells that were expanded during the response undergo apoptosis, reducing their numbers.
  • Memory Cell Formation: Not all cells become effectors; some are designated as memory cells, which live longer and can quickly respond to future encounters with the same antigen.

Fundamentals of Immunological Memory

  • Recognition: The immune system can distinguish previously encountered antigens.
  • Primary vs. Secondary Response:
  • Primary Response: Slow initiation as it starts with a limited number of lymphocytes.
  • Secondary Response: Faster and more robust due to the presence of memory cells.
  • Support: Initially, the body relies on innate immunity while the adaptive response ramps up.

Memory Cells: The Immune System's Recall Agents

  • Conservation: The body maintains a higher number of memory cells that have previously encountered their specific antigen.
  • Types: Includes both B-cells and T-cells, with T-memory cells often not clearly distinguished from T-effectors.
  • Characteristics of Memory B-Cells:
  • Selection: Among the best-performing B-cells during the primary response.
  • Adaptations: These cells have undergone class switching and somatic hypermutation, resulting in high-affinity antigen receptors.
  • Preparedness: Equipped with numerous receptors to quickly bind antigens upon re-exposure.

Quick Mobilization of Memory Cells

  • Activation: Memory cells quickly multiply and turn into new effector cells upon encountering their antigen again.
  • Efficiency: This rapid response ensures a swift and effective immune defense.

Selectivity of Memory B-Cells

  • Requirement for T-Helpers: The transformation of B-lymphocytes into memory cells depends on signals from T-helper cells and involves class switching.
  • Limitation: Memory cells predominantly form in responses involving class-switched antibodies (IgG, IgA, IgE), not typically in T-independent or primary IgM responses.

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