Antibodies (AQA A Level Biology): Revision Note
Exam code: 7402
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

Examiner Tips and Tricks
You must know that each antibody will have a different variable region with an antigen-binding site that matches one antigen or toxin produced by a pathogen. The antigen-binding site (and therefore the antibody) is specific to one antigen.
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

Examiner Tips and Tricks
Antibodies can cause pathogens to stick together, a bit like glue; this might help you remember the biological name for this process: agglutination.
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