Biology · MU Sofia

27. Functions of antibodies of different classes. Opsonization. Classical pathway of complement binding and activation.

Covers the functions of each antibody class, from IgG and IgM complement activation to IgA mucosal protection and IgE's role in mast cell degranulation, followed by opsonization. Then walks through the classical complement pathway step by step, from C1q binding to the membrane attack complex, with a picture of the pathway. It also touches on passive immunization and the main hypersensitivity types.

Inside: 2 sections · 1 labelled picture · 6 min

Topic 27 of 64 in the MU Sofia syllabus for Biology (Dental Medicine, Year 1). The free AceMed note covers it in 2 sections, about 880 words (6 min of reading).

In this note

  1. Introduction
  2. Overview of Hypersensitivity Reactions

Introduction

Functions of Antibodies

  • General Functions
  • Antigen Neutralization: Antibodies neutralize toxins by binding to them and preventing their toxic effects.
  • Immune Response Enhancement: They increase the efficiency of innate immunity by acting as adaptor molecules between antigens and innate immune components.
  • Specific Functions by Antibody Class
  • IgG:
  • Prevalence: 75% of antibodies after infection.
  • Features: Produced three weeks post-infection, quickly produced during secondary infections, crosses capillary walls and the placenta.
  • Functions: Opsonization through Fc region, activates the complement system.
  • IgM:
  • Prevalence: 10%, primary immune response.
  • Features: First to appear during immune response and B-cell differentiation, pentameric structure with a valence of 10.
  • Functions: Crude antigen binding with low affinity, strong complement system activation, remains within the bloodstream—vital for countering bacteremia.
  • IgA:
  • Prevalence: 15%, protects mucous membranes.
  • Features: Present in saliva, tears, sweat, milk, and along the respiratory and gastrointestinal tracts.
  • Functions: Forms dimers that bind microorganisms, preventing their adherence to mucosal cells; involved in transcytosis across epithelial cells.
  • IgD:
  • Function: Activates B-cells, comprising about 0.2% of antibodies.
  • IgE:
  • Prevalence: 0.002%, related to allergic reactions and parasite protection.
  • Features: Unlike IgG, it attaches to phagocytes before binding antigens.
  • Functions: Causes mast cells to degranulate upon antigen binding, contributing to allergic responses.
  • Additional Details
  • Antitoxins: Antibodies that prevent toxin actions, used in vaccines for active immunization against toxoids.
  • Mediating Inflammation
  • Cells Involved: Mast cells and basophils, equipped with receptors for Fc and IgE.
  • Process: Binding of IgE to its receptor triggers the exocytosis of granules from these cells, leading to inflammation.
  • Opsonization
  • Mechanism: Pathogens are marked for destruction by IgG antibodies or complement proteins (opsonins).
  • Interaction: Phagocytes attach to the Fc region of antibodies via Fc receptors, enhancing the efficiency of phagocytosis.

Classical Pathway of Complement Activation

  • Overview
  • The classical pathway is a key mechanism in the immune system for activating the complement system, which enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells.
  • Components and Activation Process
  • C1 Complex:
  • Composition: C1 is composed of C1q, C1r, and C1s molecules.
  • Function: C1q recognizes the Fc fragment of IgM and IgG antibodies.
  • Activation: C1q resembles a bouquet of six tulips; its heads bind to antibodies. Activation requires at least two heads to engage (either one IgM molecule or two IgG molecules close together).
  • C1r and C1s: These are proteases. The binding of C1q to antibodies activates C1r, which in turn activates C1s.
  • Cascade Activation:

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