ELISA (enzyme-linked immunosorbent assay) techniques use antibodies linked to an enzyme. Antigen–antibody reactions are detected by enzyme activity. If the indicator enzyme is present in the test well, an antigen–antibody reaction has occurred.
There are two types of ELISA tests:1. The
direct ELISA is used to detect antigens against a specific antibody bound in a test well.
2. The
indirect ELISA is used to detect antibodies against an antigen bound in a test well. For instance, this type can be used to detect the presence of antibodies. A known antigen is fixed to a small well, and the patient’s serum is added. Patient’s antibodies will react with the antigen in the well. Antihuman immunoglobulins bound to an enzyme are added to the well. The antihuman immunoglobulins will bind to the patient’s antibodies. Substrate for the enzyme is then added and a positive reaction indicating presence of the antibody in the patient’s serum is shown by the enzyme–substrate reaction.
On the other hand, a
direct ELISA test is used to detect the presence of an antigen. Antibodies are fixed to a small well, and the unknown antigen is added. If the antigen reacts with the antibodies, the antigen will be bound to the well. Antibodies specific for the antigen are then added to the well. This second layer of antibodies is bound to an enzyme. Substrate for the enzyme is then added, and a positive reaction indicating the identity of the antigen is shown by the enzyme–substrate reaction.
ELISA techniques can be used to diagnose certain diseases (i.e. HIV). Technically, it’s only value as a
diagnostic test is it’s ability to identify a particular analyte of interest, so if the particular disease of disorder is associated with the presence of some compound, ELISA can be used to detect and quantify that compound.
Here's how it's done:
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