HIV antibodies

Antibodies are protein molecules produced by the immune system in response to allergens, infectious organisms (including viruses, bacteria, fungi and parasites), and sometimes (in autoimmune disorders) the body's own components.

Infectious organisms and allergens display characteristic proteins called antigens. The immune system recognises and responds to antigens by generating corresponding antibodies. An antibody is designed to 'fit' only one particular antigen, rather like a key in a lock. (The word 'antigen' in fact derives from antibody generation.) By locking on to the antigen-bearing intruders, antibodies aim to render them harmless, to kill them outright, or to 'tag' them for destruction by other components of the immune system.

One or two weeks after initial exposure to HIV, antibodies to HIV antigens begin to appear in the blood, at concentrations which continue to increase for several more months. These antibodies persist for life, providing distinctive markers which can be identified by HIV screening tests.^{1 2 3}

Antibodies are generated in response to many different viral antigens. Antibodies to p24 antigen (see below) are generally first to appear, followed by antibodies to gp120 and gp41 – envelope proteins which appear on the outer surface of the virus. Test assays are most often geared to antibodies to these envelope proteins.

For many years, the majority of HIV diagnostic tests looked for HIV antibodies only, and some of these tests are still in use (third-generation tests). Moreover, almost all rapid tests look for antibodies only. The fourth-generation tests that are recommended for use in the UK look for both HIV antibodies and p24 antigen.

A handful of cases have been reported in which individuals have had HIV infection, but no antibodies to HIV. Whereas around 33 million people are living with HIV worldwide, there are only 25 documented cases in which the virus could be detected in a person’s blood but antibody tests were consistently negative.⁴ Common features of the reported cases are high viral loads, rapid disease progression and high mortality. In these cases, the inadequate immune system response appeared to be specific to HIV, rather than to pathogens more generally.