What are antibodies?

Antibodies are our main form of defence in the bloodstream. They are a form of protein that is produced in response to anything foreign that gets into our bloodstream — such as viruses, bacteria, or vaccines.

Antibodies are mostly limited to the humeral spaces — an almost medieval term for body fluids which are outside the actual cells of the body. These include blood, milk and other body fluids.

What do antibodies do?

Basically, antibodies stick to anything foreign that they find. The processes involved in a virus entering a body cell are complex, and having big globs of protein (antibody) hanging off a virus can be enough to block their attempted entry to cells.

This is particularly true if there are a lot of antibodies around — and if they recognise different parts of a virus and cover the surface of it.

As well as the passive function of sticking to viruses, some antibodies trigger a series of events which result in inflammation of the area around a cell area, making it generally inhospitable to bacteria and viruses.

Are antibodies killer cells?

No, but cruising around in our bloodstream are ‘killer’ cells called macrophages. When they bump into a cell or substance in the bloodstream, they need some kind of signal to determine whether they should engulf and destroy it — or whether it’s a part of the body. When a foreign body in the bloodstream has antibodies stuck to it, the killer cells take it as a signal that it is something to be destroyed.

Why don’t macrophages destroy hep C?

Hep C is a master of disguise. As the virus reproduces, it often changes its appearance ever so slightly. It’s a process called mutation and means hep C confuses our antibodies and macrophages, remaining one step ahead of them. Although we quickly eradicate lots of hepatitis C once it's identified, there are always some which have mutated, are not recognised and survive our immune response.

How do hep C antibody tests work?

Hep C antibody tests are used to see if a person has ever developed hep C antibodies.

If the test comes back positive, it means that HCV antibodies were found — proof that the virus has entered the bloodstream at some point in time.

If people are able to clear their hep C infection, they still keep their antibodies. Thus, a positive antibody test doesn’t always mean someone has a current infection.

To confirm whether a person has hep C or not, a PCR test is usually performed. These tests look for presence of the actual hep C virus.

What is the ‘window period’ for antibody tests?

Once a virus enters the body it takes a period of time before antibodies are produced. Thus, an antibody test carried out too soon following infection might return a false negative result.

With antibody tests, it takes up to 12 weeks to be sure the antibody test will return an accurate result. This is called the HCV antibody window period.

NB: Hep C diagnosis can be confirmed much earlier than 12 weeks by using PCR tests (which take only two weeks for an accurate test result).

NB: with babies, these window periods are: eight weeks with PCR, and 18 months with antibody tests.

What other blood tests are used?

Doctors will also routinely recommend liver function blood tests be done. These tests look at various substances in the blood that originate from the liver. When the substances are found in raised levels it suggests damage to liver cells.

PCR blood tests look for the actual hep C virus and are used to see whether someone has a current infection.

PCR blood tests can also measure the level of virus in the blood and are used to monitor treatment response. Further, they can also tell what subtype (genotype) of hep C virus a person has. This information is important when considering which type of treatment someone will be offered: people with genotype 1 are offered Triple Combination Treatment; people with other genotypes are offered Standard Combination Treatment.

IL28B blood tests look for your own body’s sensitivity to interferon-based antiviral treatment. They indicate whether you have an increased or decreased chance of responding to therapy.

To talk about anything in this factsheet, in NSW phone the Hepatitis Infoline on 1800 803 990 or go to www.hep.org.au

This factsheet was developed by Hepatitis NSW. It was reviewed by the Hepatitis NSW Medical and Research Advisory Panel.

Last Reviewed: 05/05/2015

Reproduced with kind permission from Hepatitis NSW.



References

Hepatitis NSW. Hepatitis factsheets. Hep C antibody testing. Last reviewed 5 May 2015. https://www.hep.org.au/factsheet-antibody-testing/ (accessed Feb 2016).