What does an indirect ELISA primarily measure?

An indirect ELISA (Enzyme-Linked Immunosorbent Assay) is specifically designed to detect and quantify the presence of antibodies in a sample. In this assay, a specific antigen is immobilized on a surface, such as a microplate. When a sample containing antibodies is added to the plate, any antibodies that recognize the antigen will bind to it.

Following this binding, a secondary antibody that is linked to an enzyme is added. This secondary antibody is specific to the antibodies of interest. Upon adding a substrate for the enzyme, a color change occurs, which can be measured spectrophotometrically. The intensity of the color correlates with the amount of antibody present in the sample. Thus, the primary focus of an indirect ELISA is to measure antibodies, making the correct answer the presence of antibodies.

Other options do not align with the primary function of an indirect ELISA. The detection of toxins or the efficiency of protein extraction relates more to different types of assays or methods. Although measuring the amount of protein can be a goal in some assays, it is not the main purpose of the indirect ELISA, which is specifically tailored to assess antibody presence rather than protein quantity directly.