Diagnostics immunology (Proceedings)

Aug 01, 2011

Immunology plays a huge role in diagnostics for infectious diseases. In addition, various assays are able to assess the immune system itself in an animal.

For infectious disease diagnostics, specific assays for pathogen detection utilize pathogen-specific antibody. There are a wide variety of assay methodologies that vary in sensitivity and specificity.

Pathogen-specific antibody is used to bind antigen in sample. Assays vary in sample type, how visualization of binding occurs, and the sensitivity and specificity. The pathogen-specific antibody may be labeled with a fluorescent tag, a radioisotope, or an enzyme (substrate addition leads to color change). Depending on the assay, samples can include tissue impressions/scrapings, pelleted cells, formalin-fixed tissue (IHC). The quality of specimen affects results. These assays generally have a fast turn-around time and are inexpensive. In addition, the orrganism does not have to remain viable. Sensitivity varies, and is generally intermediate to low; thus a positive result "rules in" but negative result may not rule out a pathogen. As these are specific assays, they will not reveal presence of other agents unless specifically targeted.

Identification of virus-specific antibody:

In these assays, antibody is the unknown rather than the antigen. It may or may not be quantified.

Methods can be divided into three categories.

     • Direct detection of antibody.
     • Detection of antibody-facilitated action.
     • Detection of antibody-facilitated inhibition.

Direct detection of antibody:

Binding of patient's antibody to capture antigen anchored on a slide or membrane.Antibody is detected through labeled anti-immunoglobulin (i.e. anti feline IgG). Examples are immunofluorescence assay, ELISA, and western blot. They vary in how the capture antigen is anchored and how the antigloublin is tagged. Some serologic assays quantitate the amount of antibody in the serum:

Detection of antibody-facilitated action:

Binding of patient's antibody to the antigen leads to an activity due to the formation of the immune complex. For example, complement fixation where binding leads to activation of complement cascade; agglutination or precipitation where the pathogen is clumped by antibody and precipitates out of solution such as agar gel immunodiffusion (AGID) assays.

Detection of antibody-facilitated inhibition:

Binding of the patient's antibody to the pathogen results in inhibition of an activity by the pathogen. Generally, these assays measure antibody to very specific surface epitopes. Hemagglutination inhibition takes advantage of the ability of some pathogens to agglutinate RBC's. Antibody binding inhibits the ability of a virus to agglutinate RBC's, and the RBC pellets out in the well. Serum Virus Neutralization assays are another example; in this assay, antibody binding inhibits the ability of a virus to infect and lyse cells in culture (antibody "neutralizes" the pathogen).