Antibody Avidity and HIV Testing

The term avidity refers to how strongly an antibody binds to its antigen.

The immune system makes antibodies in response to infection with a pathogen or certain other forms of physical insults. However, making antibodies is not a single-step process. Sometimes, the initial antibody response does not allow the body to quickly eliminate an infection. In these cases, the body will continue to develop additional antibodies against the bacteria or virus that is causing the infection. Over time, those antibodies will generally become better antibodies. Better antibodies bind either more tightly to the invader or bind to proteins that are more effective at limiting infection. The avidity of an antibody refers to how tightly it binds to its target.

It is important to distinguish between avidity and the similar term affinity. Affinity refers to the strength of any given bond between an antibody and its antigen. However, some isotypes of antibodies are multivalent and bind to multiple antigens. The strength of that overall connection is the avidity. Avidity can also be increased when an antigen with multiple binding sites interacts with a number of different antibodies.

Try to think about it as though you were measuring the intensity with which Velcro sticks to something fuzzy. The affinity is the strength with which one Velcro spike attaches to the object. The avidity is how strongly the whole piece of Velcro is capable of grabbing on.

Avidity tests are not generally ordered when doctors are investigating the body's response to a disease. However, there are certain circumstances in which avidity testing may be relevant. One of those is when doctors are trying to determine whether a newly diagnosed HIV infection is actually a new infection. Avidity tests can help determine whether the infection is new or whether the person simply hadn't been tested regularly and the early stages were missed.

Antibody Avidity and HIV Testing

The reason that avidity tests can be used to determine the length of an HIV infection is that over time, the avidity of the anti-HIV antibodies made by the immune system will improve. This method, however, is limited by prompt, effective treatment. If someone is treated well, shortly after becoming infected with HIV, higher affinity antibodies may not develop. Therefore, avidity tests may not be as useful in determining whether an HIV infection is incident or prevalent in people who were quickly given antiretroviral treatment. It is a more useful method of testing untreated populations.