Introduction to antibodies

The quickest and easiest way to find out if someone is HIV-positive is usually by testing for antibodies to the virus in blood or other body fluids. No single test is ideal for all uses. In practice, tests with different properties must be combined to get the information people need.

Beyond antibody tests, many other different tests are used in HIV medicine. Some of these are used to detect and measure HIV activity. CD4 counts and viral load tests are particularly important in taking decisions about antiretroviral treatment. These can incidentally provide a check on the accuracy of HIV antibody testing.

All antibody tests are based on the principle that antibodies are specific in what they will bind to. The 'antigen' to which an antibody binds is usually part of the surface of a protein made up of 'peptides' - particular sequences of amino-acids.

All HIV antibody tests must include HIV-related antigens. Many of them also include antibodies, which may or may not be HIV-related. The properties of each test depend on how the test antigens and any test antibodies are made and how they are used.

Some of the latest ('fourth generation') antibody tests include a test for an HIV protein called 'p24' and are therefore known as 'combined antibody/antigen tests'.

There is always a possibility of human error. Some of the tests described here are complex and depend on highly-trained and competent operators. Beyond what happens in the laboratory, good systems for taking, handling and labelling samples are vital, with checks and safeguards built into the procedures.