Antibodies (AQA A Level Biology): Revision Note

Antibodies: structure

• Antibodies are globular glycoproteins with a quaternary structure, shaped like a Y

• They are made of 2 heavy and 2 light polypeptide chains, joined by disulfide bonds

• Each chain has:

  • A constant region that is same for antibodies within one class.

    • this determines the destruction method

  • A variable region that is different in each antibody this forms the antigen-binding site

    • The antigen-binding site is specific to a particular region on the antigen called the epitope ( the part of the antigen that binds to the antibody)

    • Specificity comes from differences in the amino acid sequence of the variable region

• A hinge region (not present in all classes) is found in the heavy polypeptide chain and gives flexibility to bind antigens at different angles

Antigen-antibody complexes

• An antigen and its complementary antibody have complementary molecular shapes

  • This means that their molecular structures fit into each other

• When an antibody collides with a foreign cell that possesses non-self antigens with a complementary shape, it binds with one of the antigens

• When this occurs the two molecules combine to form an antigen-antibody complex

• Y-shaped antibodies have two antigen-binding sites, meaning that they can bind to more than one antigen at the same time

  • This can cause groups of the same pathogens to become clumped together in agglutination

• The binding of antibodies to antigens either neutralises the pathogen or acts like a marker to attract phagocytes, which engulf and destroy the pathogens

  • Due to agglutination, phagocytes can often phagocytose many pathogens at the same time, as they are all clumped together