EIAs use specific antigen-antibody binding to detect analytes present in specimens. EIAs are used to detect and measure specific proteins, peptides, hormones, drugs, and even other antibodies. A typical EIA designed to detect an antibody (for example, hepatitis B core IgM) would employ an immobilized antigen (in this case the hepatitis B core protein) on some sort of solid substrate (such as beads, a test well, or another reaction vessel). The patient's plasma is then added and any antibody in the specimen is allowed to bind the antigen. After a series of wash steps, the captured antibody can then be detected using another antibody, one that has a reporter enzyme conjugated to it. After another wash step to remove excess reporter antibody, a chromogenic substrate is added that causes a color change or a change in fluorescence absorbance. The change in color or absorbance is proportional to the amount of patient antibody present in the specimen. EIAs can be either quantitative or qualitative. EIAs can be designed to detect antibody (as in the example and figure shown) or they can utilize a capture antibody which would then bind a specific antigen, rather than an antibody, in the patient's specimen.
