How Do Antibodies Work? AQA A Level Biology

Antibodies work by binding to specific antigens on a pathogen. This forms antigen–antibody complexes that help the immune system remove the pathogen. Antibodies do not usually kill pathogens directly.

How does an antibody recognise a pathogen?

An antigen is a molecule, often on the surface of a pathogen, that triggers an immune response. An antibody is a protein made by a B lymphocyte. The ends of an antibody have variable regions with binding sites whose shapes are complementary to a particular antigen. This is why an antibody binds to a specific antigen rather than to every pathogen.

The antibody's specific three-dimensional shape depends on its amino acid sequence and how the protein folds. Each antibody has two antigen-binding sites, so it can bind two matching antigens.

What happens after binding?

An antigen–antibody complex is an antibody joined to its matching antigen. Binding can mark a pathogen for uptake: the antibody acts as a label that phagocytes can recognise.

When antibodies bind to antigens on different pathogens, they can link the pathogens together. This is called agglutination. The resulting clumps are easier for phagocytes to engulf and destroy by phagocytosis.

For example, if several bacteria carry the same surface antigen, antibodies can attach to them and cause agglutination. A phagocyte then engulfs the clump and uses lysozymes to break down the bacteria. The antibody has assisted removal, not done the digestion itself.

B lymphocytes with receptors complementary to the antigen are selected and divide to produce a clone. Some become plasma cells, which secrete large quantities of antibody. Others become memory cells. If the same antigen enters the body again, memory cells help produce a faster, larger secondary response.

Exam mistake to avoid: Do not write that antibodies themselves engulf or kill pathogens. Antibodies bind antigens and help phagocytes remove the pathogens.