Immunoassays for diagnosis of infectious diseases

Immunoassays have a number of advantages for infectious disease diagnosis. These advantages are based on the sensitivity and specificity inherent in antigen–antibody interactions. While there are a number of immunoassay systems that can be used for the direct detection of infectious antigens in human body fluids, solid-phase immunoassay techniques have attained widespread usage for this purpose. There are a number of possible solutions to the problems inherent in the direct linkage of enzymes to immunoglobulins. For example, immunoassays can be performed in indirect formats in which an unlabeled primary antimicrobial immunoglobulin is reacted with immobilized antigens and the resultant complex is quantified by an enzyme-conjugated second antibody. More importantly, the species specificity of many anti-immunoglobulin conjugates is not absolute. Nonspecific binding can generate high background levels or, in certain cases, false-positive reactions. The same problem is inherent in the use of other immunoglobulin-binding macromolecules such as staphylococcal protein A.