Key notes A varjety of assays have been developed which provide specitic qualitative and quantitative measurement of Ag or Ab, both of which are often of considerable research and clinical relevance. Ab to an organism in the serum of a patient demonstrates infe- ction by the organism. Ab with defined specificity is used to determine the presence of disease associated antigens in a patient. As tools in molecular and cellular research, Abs permit localization and characterization ofA variety of assays have been developed which provide specific qualitative and quantitative measurement of Ag or Ab, both of which are often of considerable research and clinical relevance. Ab to an organism in the serum of a patient demonstrates infe￾ction by the organism. Ab with defined specificity is used to determine the presence of disease associated antigens in a patient. As tools in molecular and cellular research, Abs permit localization and characterization of Ags. Key notes