ELISA test plate. Technician using a pipette to transfer blood serum into a multi-well sample tray (white) for an enzyme-linked immunosorbent assay (ELISA) blood test. This test checks for the presence of proteins such as antibodies and antigens in the blood. Antibodies are produced by the immune system in response to antigens from disease-causing organisms, such as hepatitis B, rubella, HIV and chlamydia. A multi-well sample tray allows analysts to run many samples simultaneously. The technician wears gloves and uses a different pipette tip (in orange tray) for each sample, to ensure that samples do not become contaminated.