The Molecules of HIV

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ELISA

ELISA is the Enzyme-Linked Immunoadsorbent Assay. It's a sensitive immunoassay used to detect a specific protein – for example antibodies to HIV borne in a sample of blood.

There are a few methods for performing an ELISA. Here is one common one, and the one relevant to detecting HIV antibodies:

1. Put a specific antigen protein (i.e. prepared HIV protein) in a plastic container. The protein molecules bind (adsorb) to the plastic by hydrophobic or other attraction.
2. Add the serum sample from the patient. If the patient serum contains HIV antibodies, these will attach to the HIV antigens already present.
3. Wash the container gently, so that all the unbound components of the patient serum are washed away.
4. Add an antibody which is raised to recognise the human antibody, and of which each molecule is covalently linked with a molecule of an enzyme such as alkaline phosphatase or peroxidase - any enzyme which can catalyse a chromogenic reaction.
5. Wash the container gently, so that any unbound test antibody from the previous step is washed away.
6. Add the reagents which are the inputs to the chromogenic reaction. These reagents will only react in the presence of the enzyme - that is, where our test antibodies have bound to the plastic container via the antibody/antigen complex.
7. The colour produced in the container is an indication of the amount of antibody found. Normally the ELISA is performed in a multi-welled container, and a positive result comes when the amount of wells showing the appropriate colour change is a certain proportion.

False positives can occur - for example antibodies induced by a recent flu jab can cause a positive result - and so although ELISA's an important diagnostic test for HIV, it should be followed up with a Western blot test.

You may also want to read about Tests of HIV progression.

Written by
Dan Stowell
(©2002-2006)