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13.4.1.2. Antibodies

Antibodies

The antibodies, also called immunoglobulins, are proteins with a small carbohydrate part. The structure of the antibody is Y-shaped. Each antibody molecule is made up of four polypeptide chains linked together by disulfide bonds (-SS-) linked to an oligosaccharide. Therefore, these  proteins have a quaternary structure, acquiring a "Y" shape. 

  • Two chains are longer (heavy or H chains), identical to each other.
  • Two shorter chains (light or L chains), and also identical.

Each antibody molecule has two regions:

  • Constant region: it is the same for each class of antibody, but different from each other. It corresponds to the base of the Y and the lower part of the arms. It belongs to the C-terminal end of polypeptide chains, which serves to be recognized by lymphocytes.
  • Variable region, in the upper part of the Y, where the amino termini of the polypeptide chains meet. In this zone the paratope (formed by a few amino acids) is distinguished, which is the place of union with the antigenic determinants or epitope .

Thus, each immunoglobulin (or antibody) can bind to two antigen molecules, which is why it is said to be bivalent or to have valence 2. Some antibodies associate to form dimers or pentamers, with two or five monomers, respectively, where they can bind to more antigen molecules.

The carbohydrate part of the antibody is made up of polysaccharide chains covalently linked to the constant region of the long chainsIts function is not clear, but it seems that it is involved in the secretion of immunoglobulin or in its protection against enzymatic attack.



Fundamental Ideas About Antibodies

The antibodies or immunoglobulins have these characteristics:

  • They are glycoproteins created by B lymphocytes
  • Y-shaped structure:
    • Two chains are longer (heavy or H chains), identical to each other.
    • Two shorter chains (light or L chains), and also identical.
  • Each antibody molecule has  two regions:
    • Constant region.
    • Variable region, in the upper part of the Y, specific for each antigen.