Why is an antibody specific to one antigen

The HI test measures how well these antibodies recognize and bind to other influenza viruses such as, influenza viruses isolated from flu patients. If the ferret antibodies that resulted from exposure to the vaccine virus recognize and bind well to the influenza virus from a sick patient, this indicates that the vaccine virus is antigenically similar to the influenza virus obtained from the sick patient.

This finding has implications for how well the vaccine might work in people. As previously mentioned, the influenza viruses used in the HI test are obtained from sick people. CDC and other WHO collaborating centers collect specimens from people all over the world to track which influenza viruses are infecting humans and to monitor how these viruses are changing.

They are used in the HI test because influenza viruses bind to them. Normally, RBCs in a solution will sink to the bottom of the assay well and form a red dot at the bottom Figure 2A.

This keeps the RBCs suspended in solution instead of sinking to the bottom and forming the red dot. Row A shows that in the absence of virus, RBCs in a solution will sink to the bottom of a microtiter plate well and look like a red dot. Row B shows that influenza viruses will bind to RBCs when placed in the same solution. When antibodies are pre-mixed with influenza virus followed by RBCs, the antibodies will bind to influenza virus antigens that they recognize, covering the virus so that its HA surface proteins can no longer bind to RBCs Figure 2C.

The reaction between the antibody and the virus inhibits i. When the antibodies do recognize and bind to the influenza virus in the solution, this shows that the vaccine virus like the one the ferret was infected with is similar to the influenza virus obtained from the sick patient. When a circulating influenza virus is antigenically different from a vaccine, the antibodies developed in response to the vaccine virus may not recognize and bind this virus. In the HI test, this will cause hemagglutination to occur see Figure 2B.

Circulating influenza viruses tested via the HI test are typically obtained from respiratory samples collected from sick patients. The HI test assesses the degree of antigenic similarity between two viruses using a scale based on antibody dilution.

As previously mentioned, the HI test is performed using a microtiter plate. The microtiter plate contains rows and columns of wells i. Dilutions are marked across the top of the microtiter plate. These dilutions function as a scale for assessing antigenic similarity and immune response.

By testing the ability of greater dilutions of antibody to prevent hemagglutination, scientists measure how well those antibodies recognize and bind to an influenza virus. The higher the dilution, the fewer antibodies are needed to block hemagglutination and the more antigenically similar the two viruses being compared are to each other.

The highest dilution of antibody that results in hemagglutination inhibition of the test virus is considered an HI titer Figure 3. This virus has an HI titer of , which means that the greatest dilution of antibody that still blocked hemagglutination from occurring was at dilution. At this dilution, the antibodies were still capable of recognizing and binding to the antigens on the virus.

When CDC antigenically characterizes influenza viruses to inform decisions on the formulation of the seasonal flu vaccine, the HI test is used to compare currently circulating viruses rows B and C with vaccine viruses row A. This allows scientists to quickly determine if a virus used in the seasonal flu vaccine is antigenically similar to circulating influenza viruses and therefore capable of producing an immune response against them.

This is equivalent to a two-well i. Circulating viruses that are antigenically dissimilar i. Antigenic characterization gives important information about whether a vaccine made using a specific vaccine virus will protect against circulating influenza viruses, but there are several limitations to antigenic characterization test methodology, which are described below. Right now, most flu vaccines are made using viruses grown in chicken eggs. As human influenza viruses adapt to grow in eggs, genetic changes can occur in the viruses.

Egg-adapted changes have become a particular problem for selection of candidate vaccine viruses CVVs for the influenza A H3N2 virus component of flu vaccine made using egg-based production technology. Influenza A H3N2 viruses tend to grow less well in chicken eggs than other influenza viruses e. A H1N1 pdm09 , and they also are more prone to egg-adapted changes. Such changes can reduce the immune protection provided by the flu vaccine against circulating A H3N2 viruses.

Using current egg-based vaccine production technology, it takes about six months from the time that a vaccine virus is chosen i. Because influenza viruses are constantly changing, circulating influenza viruses may change during this six month period. If these genetic changes cause antigenic changes, it could mean that antibodies created through vaccination may not recognize and inactivate circulating influenza viruses.

New technologies that shorten the manufacturing time could reduce the chances of significant antigenic changes occurring in circulating viruses before influenza vaccines become available each year: for example, cell-based and recombinant flu vaccines. Use of the HI test to assess the similarity of circulating influenza viruses to vaccine viruses involves obtaining antibodies from animals particularly ferrets.

Ferrets are often used as an animal model for influenza infection because they can be easily infected with human influenza viruses and experience similar signs and symptoms e. Prior to infection, ferret serum is pre-screened to determine that it does not contain antibodies to circulating influenza viruses.

When ferrets are infected with a live influenza virus, they typically produce strong antibody responses against that particular virus. Therefore, ferret antisera are commonly used to detect antigenic differences between influenza viruses.

However, there are differences between the immune responses of ferrets and humans which need to be considered when assessing the antigenic properties of influenza viruses. More information on use of human sera is available at human serology and flu. The B-cells respond to the vaccine by reproducing themselves to form an army of cells that are programmed to react to the antigens in the vaccine.

The antibodies created by the vaccine lie dormant in your body until you contract an infection from that antigen, and then they are called to action. If you contract an infection, antibodies called memory B cells quickly reproduce and make the specific antibodies you need to destroy that antigen. Antigens trigger your immune system to launch an antibody response. Specific antibodies detect specific antigens.

This means each antibody wages war against one target antigen. Once antibodies detect antigens, they bind and neutralize them. It launches fights against the antigen should it attempt to attack your body again. The distinct functions of antigens and antibodies are used to create tests and vaccines that help detect and combat illness and disease. Blood typing…. A GAD antibody test can help your doctor determine if you have type 1 or 2 diabetes.

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Learn more…. Health Conditions Discover Plan Connect. Antigen Antibody Vaccinations Antibody testing Bottom line Antigens and antibodies play vital but distinct roles in illness and disease. What is an antigen? What is an antibody? How are antigens and antibodies used in vaccinations? The bottom line. Read this next.