A sandwich ELISA is used to detect antigen by capturing it between two antibodies to quantify it.

• A. A sandwich ELISA detects antigen by capturing it between two antibodies to quantify it.
• B. A direct labeling assay where antigen is tagged and measured by color change.
• C. A competitive binding assay where antigen competes with a labeled antigen.
• D. A single antibody directly coating the plate and detecting with another labeled antibody.

Answer: (A)

A sandwich ELISA relies on two antibodies to form a sandwich around the target antigen. First, a capture antibody is attached to the plate and binds one epitope on the antigen. Then, after washing away everything unbound, a second antibody binds another epitope on the same antigen. This second antibody is typically linked to an enzyme that, when a substrate is added, produces a color change. The signal is proportional to how much antigen is present, which makes this format highly specific and sensitive. It requires the antigen to have at least two distinct binding sites, which helps distinguish the target from nonspecific background.

Direct labeling assays, where the antigen itself is tagged and detected with a single antibody, don’t use this two-antibody sandwich approach and are generally less specific. Competitive binding formats involve the antigen competing with a labeled antigen for antibody binding and produce signals that inversely relate to antigen concentration, making them a different type of assay. The description involving a single coating antibody with a second labeled antibody could resemble a sandwich but is less explicit about the antigen being captured between two different antibodies, which is the defining feature of a sandwich ELISA.