Formation and function of antibodies

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Antibodies are necessary in providing immunity against pathogens and assisting other parts of the immune system

Antibodies[✎ edit | edit source]

Structure of an antibody
  • Antibodies are gamma globulins (20% of total plasma proteins)
  • Each B-cell clone makes antibody molecules with a specific antigen-binding site
  • The antigen-binding site binds to the membrane and serve as receptors for antigens specific to its site
  • When an antigen binds to the receptors of the antibody on the B-cell, the B-cell activates to multiply and mature into memory cells or into an antibody-secreting cells, which produce antibodies with the same antigen-binding site
  • Antibodies are made of light and heavy polypeptide chains - each heavy chain is paralleled by a light chain at one of its ends
  • The light chain and part of the heavy chains form the variable region; the remainder of the heavy chains form the constant region
  • The variable region is different in each specific antibody and attaches specifically to an antigen
  • The constant region determines the other properties of the antibody e.g: diffusivity in tissues, adherence, attachment to the complement complex, etc.
  • Each antibody is specific to a particular antigen due to complementary amino acid sequences
  • Different parts of the variable regions are produced due to separate gene segments (VDJ sequences), which are recombinated during B-cell differentiation

Classes of Antibodies[✎ edit | edit source]

IgG[✎ edit | edit source]

  • They are bivalent antibodies and comprise 75% of all antibodies
  • They are produced during the secondary response, identifies micro-organisms for phagocytosis, activates complement system and binds to macrophage receptors
  • IgG-coated foreign cells are attacked by killer cells
  • IgG can pass from mother to foetus via the placenta

IgA[✎ edit | edit source]

  • They are present in secretions (milk, saliva, tears, respiratory and intestinal secretions)
  • They are dimeric
  • They agglutinate infectious agents in secretions as well as having anti-viral action

IgM[✎ edit | edit source]

  • They have 10 binding sites
  • Activates complement
  • All B-cells initially make IgM antibodies
  • Stimulates phagocytosis and complement reaction

IgE[✎ edit | edit source]

  • They have 10 binding sites and bind to receptors on the surface of mast cells and basophils
  • They trigger the secretion of serotonin and histamine by these cells, which increase the permeability of vessels for leukocytes, antibodies and complement components during inflammation

IgD[✎ edit | edit source]

  • They are rarely secreted by B-cells

Functions of Antibodies[✎ edit | edit source]

Due to the multivalent nature of the antibodies and the multiple antigen sites on most pathogens, antibodies can inactivate antigens in many ways:-

  • Agglutination - bacteria are bonded through their antigens into a clump
  • Precipitation - soluble antigens and the binded antibodies become insoluble and precipitate
  • Neutralisation - antibodies cover the toxic site of the antigen
  • Lysis - few antibodies are potent enough to directly attack the pathogen membrane and rupture the cell

Effects of the Complement System[✎ edit | edit source]

  • When an antibody binds with an antigen, a specific site on the constant region of the antibody binds with the C1 molecule of the complement system and activates this system (one antigen-antibody complex can activate many C1 molecules)
  • The C1 molecule (enzyme) activates increasing numbers of other enzymes in the system (amplification)
  • Multiple end-products have different effects on the pathogens e.g: opsonisation followed by phagocytosis, lysis, agglutination, neutralisation of viruses, chemotaxis, activation of mast cells and basophils and inflammatory effects

Links[✎ edit | edit source]

Bibliography[✎ edit | edit source]

HALL, John E. Guyton and Hall Textbook of Medical Physiology. 11th Edition edition. 2006. ISBN 978-1-4160-4574-8.