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Overview
 

 
Introduction
  • Antibodies are immunologically active circulating proteins that
    • are composed of two heavy chains paired with two light chains
    • serve as a primary component of humoral immunity
    • are produced by B-cells that can further
      • differentiate into plasma cells that specialize in secreting antibodies
      • mature to make antibodies with higher affinity
      • remain dormant as memory cells
    • bind antigens from a wide variety of pathogens
    • are also known as immunoglobulins (Ig)
  • Antibodies are able to fight infections through multiple mechanisms including
    • opsonization of the surface of the pathogen leading to
      • phagocytosis by innate immune cells like macrophages
      • cytotoxicity by triggering release of toxic compounds by innate immune cells
    • neutralization of pathogens and viruses by
      • blocking interaction of pathogenic proteins with host receptors
      • inactivating virulence factors expressed by pathogens
    • activation of the complement cascade through the classical pathway
Antibody Structure
  • Antibodies are composed of two heavy chains paired with two light chains
  • Together these chains create distinct regions of the antibody such as
    • the constant fragment (Fc)
    • two identical variable antigen binding fragments (Fab)
  • These regions differ in both structure and function
Differences Between Antibody Regions
Feature Antibody Binding (Fab)
Constant (Fc)
Composition
  • One heavy and one light chain
  • Linked by disulfide bonds
  • Two heavy chains
  • Linked by disulfide bonds
  • Attached to carbohydrate chains
Terminus
  • N-terminus of protein chains
  • C-terminus of protein chains
Function
  • Binds to antigen in specific manner      
  • Binds to complement proteins
  • Binds to effector regions of innate immune cells       
Variability
  • Site of idiotype diversity
  • Changed in affinity maturation
  • Unique for every antibody
  • Site of isotype diversity
  • Changed in isotype switching 
  • 5 types that are shared among all antibodies
Antibody Variation and Diversity
 

 
  • Antibodies are able to fight an incredible range of infections because of 
    • idiotype diversity which
      • governs what antigens can be recognized by antibodies
      • is generated by multiple diversity mechanisms including
        • random recombination of VDJ regions of antibody coding regions
        • random addition of nucleotides to hypervariable regions by TdT
        • random assortment of heavy chains with light chains
        • affinity maturation through somatic hypermutation after antigen exposure
      • ensures that any moiety can be recognized by the variable region of an antibody
    • isotype diversity through five types of constant regions
 
Antibody Isotypes
Feature IgA IgD
IgE IgM
IgG
Constant chain type
  • α chain
  • δ chain
  • ε chain
  • μ chain
  • γ chain
Concentration
  • High in mucus membranes and in secretions
  •  Low
  •  Low
  • High in serum during early response
  • High in serum during late response
Valence
  • Dimer
  • Monomer
  • Monomer
  • Pentamer
  • Monomer 
Function
  • Mucosal immunity
  • Transported into mucosal lumens by poly Ig
  • Present on B-cell surfaces
  • Unclear role
  • Defence against parasites 
  • Mediates allergies
  • Low affinity
  • Main antibody in early response
  • High affinity
  • Main antibody in late reponse
Recognized by
  • Innate immune cells
  • Complement
  •  Unknown
  • Mast cells
  • Basophils
  • Innate immune cells
  • Phagocytes
  • Complement

Reference

 

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