Definition of antigen. The effect of antigen variability on disease and…

The idea

An antigen is a molecule, usually a protein or glycoprotein and usually found on a cell surface, that is recognised as foreign and stimulates an immune response. Recognition depends entirely on shape: the receptor on a lymphocyte and the binding site of an antibody have a tertiary structure complementary to one specific antigen. Antigen variability follows from mutation in the pathogen's DNA or RNA. A change in the base sequence changes the sequence of amino acids in the antigen, which changes its tertiary structure, so the antibodies and memory cells left from a previous infection are no longer complementary to it. They cannot bind, no secondary response is triggered, and the immune system must mount a slow primary response all over again, so the person becomes ill a second time.

What earns the marks

• Defining an antigen as a molecule, usually a protein or glycoprotein, recognised as foreign or non-self, that stimulates an immune response.
• Mutation changing the base sequence and therefore the amino acid sequence of the antigen.
• The changed tertiary structure no longer being complementary to existing antibodies or memory cell receptors.
• The consequence that memory cells cannot bind, so a primary response occurs again and symptoms appear.
• Applying this to prevention, such as an influenza vaccine having to be changed each year.

Common mistakes and how to put them right

• Defining an antigen as 'a pathogen' or 'a germ' rather than as a molecule carried by the pathogen.

• Writing that the pathogen changes its antibodies.

  Put it right: Pathogens carry antigens and only plasma cells make antibodies.

• Placing the mutation in the infected person rather than in the pathogen.

• Claiming antigen variability destroys memory cells, when the memory cells still exist but are no longer complementary.

• Saying 'the vaccine stops working' without explaining the loss of complementary shape.

Exam tips

• Learn the definition as a single sentence containing three elements: type of molecule, recognised as foreign, stimulates an immune response.
• Finish the whole chain in variability questions: mutation, different amino acid sequence, different tertiary structure, not complementary, no secondary response, symptoms.
• Write 'not complementary' rather than 'does not fit'; the complementary-shape wording is what mark schemes look for.