The keystone of an immunoassay is the reaction between the antigen and the antibody. The easiest way to describe this process is to imagine the antigen as a key and the antibody as a lock. Only one key will properly fit and operate the lock (Fig. 22.3). Thus, the antibody will selectively recognise the antigen among the myriad of structurally related substances that are present in the sample. The selectivity of an immunoassay is, in consequence, predominantly dependent on the specificity of the reaction between the antigen and antibody.
