A rapid enzyme immunochemical method for determining the presence of an antigen, antibody, or hapten in the blood. In EIA an antigen or antibody is bound to an enzyme (e.g., horseradish peroxidase or alkaline phosphatase). The resulting molecule can bind specific immunological targets in body fluid samples and highlight their presence enzymatically. EIA, formerly known as enzyme-linked immunosorbent assay (ELISA), is used as one of the primary diagnostic tests for many infectious diseases, including treponema pallidum (the spirochete that causes syphilis) and the human immunodeficiency virus (HIV).