What is the purpose of a neutralization assay in clinical laboratory testing?

• A. Measure total antibody concentration
• B. Detect immune complexes
• C. Measure the functional capacity of antibodies to neutralize a toxin or pathogen
• D. Assess T-cell cytotoxicity

Answer: (C)

Neutralization assays are about the functional activity of antibodies—their ability to stop a toxin or a pathogen from causing infection or damage. This isn’t just about how much antibody is present; it’s about whether those antibodies can block the pathogen from entering cells or the toxin from acting. In practice, patient serum is mixed with a fixed amount of virus or toxin and then exposed to susceptible cells. If neutralizing antibodies are present, they prevent infection or toxin effect, so you see little to no damage or cytopathic effect, and the degree of inhibition tells you the neutralizing antibody titer. This is why neutralization assays are used to gauge protective, functional antibody responses, such as after vaccination or infection, rather than simply measuring total antibody levels.

This differs from measuring total antibody concentration, which just tells you how much antibody is there without indicating whether it can block infection. It also isn’t about detecting immune complexes, which is a different type of test, or about T-cell cytotoxicity, which assesses cellular immune responses rather than antibodies.