Antibody and Antigen Tests

What are antibodies and antigens?

Antibodies are small proteins that circulate in the bloodstream. They are part of the immune (defence) system and are sometimes called immunoglobulins. They are made by B-lymphocytes (a type of white blood cell). Antibodies attach to proteins and other chemicals in the body which they recognise to be foreign (not normally found in the body). The foreign proteins and chemicals that antibodies attach to are called antigens.

Antibodies and infection

Antibodies help to defend us against infection. Bacteria, viruses, and other germs have antigen proteins on their surface which the immune system recognises to be foreign. So, when we have certain infections, B-lymphocytes make lots of antibodies which attach to the infecting germs. This often destroys the germ, or attracts other parts of the immune system (white blood cells, etc) which come and destroy the germ.

Each antigen has a different shape, and so requires a different antibody to attach to it. For example, the antibody that is made to attach to the measles virus is different to the antibody that is made to attach to the chickenpox virus. Therefore, antibodies to measles will not be able to defend you against chickenpox.

Once we have been infected with a particular germ, the immune system 'remembers' the particular antigen on the germ, and the antibody needed to attach to it. So, if you come in contact with the same germ in the future, the B-lymphocytes quickly make lots of the appropriate antibody which can block and fight off the germ before it causes infection. This is why you normally have certain infections only once, as from then on you are immune to further infection.

This is also how immunisation works. That is - you are given a small or 'killed' dose of a germ. The body makes antibodies to the antigen on its surface. So, from then on, if you come into contact with the same germ, your body quickly makes lots of antibody to attach to the germ and destroy it.

Auto-antibodies

Auto-antibodies occur in people with some autoimmune diseases. In these conditions you make antibodies to proteins or structures which occur in a part of your body. The immune system 'mistakes' a protein in your body to be a foreign antigen. It then makes antibodies which attach to this protein. This can cause damage or other problems to the affected part of the body. For example, the common cause of an underactive thyroid gland is due to an autoimmune problem. Antibodies attach to cells of the thyroid gland, which stops them from making thyroid hormone.

Some antibody tests

Various antibodies can be detected and measured in blood samples, and sometimes in other samples such as saliva. Scientists are discovering more antibodies all the time. The results of some antibody tests are thought to be 'diagnostic' for particular illnesses. This means that, if you test positive for a particular antibody, this proves that you have a certain condition. However sometimes a positive test means it is not certain, but much more likely, you have a particular illness. Antibody tests may be done for various reasons:

To detect infection

Tests to detect certain antibodies can help to diagnose some types of infections. However, when you are newly infected with a germ, it takes several days for the level of antibody to rise and to be detected. Therefore, antibody tests are often not useful to diagnose a new infection quickly. Other tests, such as microscopy and culture, and antigen tests, are often quicker and more useful in the situation where a quick diagnosis is needed for a new illness.

However, some antibody tests are useful to diagnose a persisting infection, or to confirm that you have been infected in the past and are now immune. For example, the test to check that pregnant women are immune to rubella (German measles) is an antibody test.

To diagnose autoimmune disorders

Conditions such as thyroid disorders, Addison's disease, pernicious anaemia, primary biliary cirrhosis, and some other autoimmune conditions have particular auto-antibodies that can be detected in a blood sample. In some autoimmune skin conditions, an antibody can be detected from a sample of the skin. (Antibodies can be identified attached to structures in the skin.)

To diagnose certain other conditions

For example:

• Myeloma is a condition where the bone marrow makes too many lymphocytes which make abnormal quantities of antibodies. These can be detected in the blood to diagnose this condition.
• Some immune deficiency conditions have low levels of antibodies. Therefore, if you have recurring or persisting infections, you may have tests to see if you lack certain types of antibody.
• Antibody tests are sometimes helpful in diagnosing some cancers and allergies.

Antigen tests

Some tests can identify the antigen on the surface of some bacteria and some other germs. These tests can be useful to detect an infecting germ quickly without the need to grow (culture) it, or to see it under a microscope. For example, a test on a sample of faeces (stool) can detect the antigen on the surface of a bacterium called Helicobacter pylori. This bacterium can infect the stomach and duodenum to cause duodenal and stomach ulcers. Detecting the antigen in the faeces confirms that you have this bacterium in your gut.

Various other antigen tests help to diagnose certain other infections and conditions.