Biology · MU Sofia

26. Adaptive immunity. Humoral immunity. Structure of antibodies. Polyclonal and monoclonal antibodies. Antigenic determinants recognized by antibodies. Antigens and haptens.

Explains the structure of antibodies: four polypeptide chains forming a Y-shaped molecule, with variable and constant domains, the antigen-binding sites at the arm tips, and the complementary determining regions. Covers the kappa and lambda light chain types, the five heavy chain isotypes, and how IgA and IgM form dimers and pentamers. Compares polyclonal and monoclonal antibodies, then describes epitopes and the weak non-covalent interactions that bind antibody to antigen.

Inside: 3 sections · 6 labelled pictures · 3 min

Topic 26 of 64 in the MU Sofia syllabus for Biology (Dental Medicine, Year 1). The free AceMed note covers it in 3 sections, about 413 words (3 min of reading).

In this note

  1. Introduction
  2. Polyclonal vs. Monoclonal Antibodies
  3. Antibody-Antigen Interaction

Introduction

Structure of Antibodies & Epitopes

  • General Structure
  • Complexity: Antibodies are quaternary structures held together by disulfide bonds.
  • Components: Composed of four polypeptide chains—two heavy (H) and two light (L) chains.
  • Shape: Symmetrical and Y-shaped, with a flexible hinge region that allows bending.
  • Antigen-Binding Sites: Located at the tips of the Y arms.
  • Solubility: The lower part of the Y is hydrophilic in soluble antibodies.
  • Chains and Domains
  • Composition: Each chain consists of domains with similar, though not identical, structures supported by intrachain disulfide bonds.
  • Glycosylation: Immunoglobulins are glycosylated to enhance functionality.
  • Light Chain Domains: Comprises two domains; one variable (VL) and one constant (CL).
  • Heavy Chain Domains: Typically has four or five domains; one variable (VH) and the rest constant (CH).
  • Specificity and Binding
  • Variable Domains: Located in the N-terminal position, variable domains differ between antibodies and determine specificity for antigens.
  • Antigen-Binding Formation: VH and VL together create the antigen-binding site.
  • Structure of Domains: Built predominantly on beta-sheets, except the hinge region which remains extended.
  • Hypervariable Regions: Variable domains contain three loop-shaped hypervariable regions which, along with the light chain, form the antigen-binding site known as the Complementary Determining Regions (CDR).
  • Framework Regions: Surrounding the CDRs, these regions form beta sheets that maintain the CDRs in positions optimal for antigen binding.
  • Constant Regions and Isotypes
  • Genetic Encoding: The constant part of the antibody is encoded variably by the genotype, forming different isotypes.
  • Light Chain Isotypes: Two types—kappa and lambda, which perform similar functions.
  • Heavy Chain Isotypes: Five types—gamma, alpha, mu, delta, and epsilon, corresponding to IgG, IgA, IgM, IgD, and IgE, each differing in function and structure. IgM and IgE have an extra domain in their heavy chains.
  • Molecular Forms
  • Basic Immunoglobulin Monomer: Consists of two light and two heavy chains.
  • IgA and IgM Structures:
  • IgA: Forms a dimer consisting of two basic monomers.
  • IgM: Forms a pentamer consisting of five basic monomers, linked by a J ("joining") chain and additional disulfide bonds in the case of IgM.

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