ELISA stands for 'enzyme-linked immunosorbent assay'. Immunosorbent means it uses the principle of an antibody binding to an antigen. Enzyme-linked has been explained in Making antibodies visible.

When blood is taken for an ELISA test, a clear fluid, or serum, which contains any antibodies, is first separated from the red blood cells in the blood.

This serum is added to the substrate on which the antigen is fixed and incubated. If there are antibodies in the serum that recognise the antigen, they will bind to the antigen.

The substrate is then washed, to remove any antibodies that are NOT bound to the antigen.

The enzyme-linked antibody is then added and incubated, to bind to any antibodies that are still fixed to the antigen.

Once more, the substrate is washed to remove any enzyme-linked antibodies that are not bound to it.

Finally, a solution of the chemical on which the enzyme acts is added.

If the enzyme-linked antibody is present, a reaction occurs and a change of colour is seen. This is taken as a positive result.

If the enzyme-linked antibody has all been washed away, no reaction occurs. This is taken as a negative result.