Figure 1. Schematic representation of the immunoglobulin G molecule: red, heavy chain; orange, light chain; dotted circle, paratope region.

Figure 2. Diagrammatic representation of the typical relationship between the amount of immunocomplex formed when a variable amount of protein antigen reacts with a fixed amount of antibody (—). This is known as the Heidelberger–Kendall curve. The left hand sector (---) denotes the type of calibration curve found in an immunometric excess-reagent assay. The right hand sector () denotes the type of calibration curve found in a competitive, limited-reagent assay.

Figure 3. Schematic representation of a competitive limited-reagent (a) and immunometric excess-reagent (b) heterogeneous immunoassay.

Figure 4. Schematic representation of a homogeneous immunoassay in which, when the sample antigen is present, it binds to the antibody, thereby allowing access of a substrate to the active site of an enzyme labelled with the antigen. In the absence of sample antigen, the antibody binds to the antigen attached to the enzyme, inhibiting access to the substrate, thereby effectively modulating the activity. The split box substrate denotes that the enzyme is acting to split the substrate.

Figure 5. Schematic representation of an immunochromatographic device.