Immunoglobulins, or antibodies, are proteins synthesized by an animal in response to the presence of a foreign substance, the antigen. These proteins, naturally designed with an extraordinary specificity and binding affinity for a given antigen, are used as analytical reagents in diagnostics, in environmental and food tests (immunoassays and immunosensors), and in separations (immunoaffinity chromatography). From the analytical point of view, the antibody is considered as either the recognition element or the receptor when applied to tests or chromatography, respectively. Accordingly, the antigen acts as the analyte that has to be detected or the ligand to be separated. Therefore, the naturally occurring immunoreaction is transformed into a selector-analyte or receptor-ligand interaction for the purpose of the analytical applications. Interestingly, almost all these methods are based on immunochemical reactions carried out on a solid surface, with which either the antibody or the antigen is associated. Solid-phase methods provide a simple way of separating bound and free reactants, and they can be combined with any detecting technique available for surface analysis. Fig. 1 shows a typical scheme for monitoring the formation of the antigen-antibody complex at the interface when the selector (or receptor) is attached to the solid surface. In the development of solid phase methods the main goal is to achieve a stable attachment of immunoglobulin molecules at the solid surface without disrupting or masking the biological function of the protein, i.e., its selectivity and specificity. Afterward, nonspecific interactions between the soluble analyte (or ligand) and the solid phase are inhibited or blocked to assure that only the immunoreaction will produce a detectable change in the system.