Immunoassay uses antibody-antigen reactions to identify and quantify unknown analytes in solutions. An antigen is any molecule recognized by the immune system, especially antibodies, while an antibody is a large Y-shaped protein that binds specifically to epitopes on antigens. For a substance to stimulate antibody production and be called an immunogen, it typically needs to have a molecular weight over 5-10 kDa; smaller molecules called haptens must be attached to carrier molecules to elicit antibodies.

1. Immunoassay
Immunoassay is a biochemical method that identifies and quantifies unknown
analytes (protein, lipid, nucleic acid etc.) in solution (serum, urine etc.) using
antibody-antigen reactions. There are many different formats and variations of an
immunoassay, but the key point is still specific antibody-antigen recognition. What is
an antigen and an antibody? Why can an antibody bind to an antigen?
An antigen is a molecule that is recognized by the immune system, particularly by
antibodies. Proteins, polysaccharides, pesticides, antibiotics, toxins, and hormones
all can be antigens. But not all antigens can stimulate the immune system to
generate antibodies. An antigen that can elicit an immune response, and particularly
antibody synthesis is called an immunogen. Proteins with molecular weights higher
than 5-10 kDa are immunogens, while small peptides, pesticides, antibiotics, or
hormones are not. These low-molecular-weight substances are called haptens and
must be chemically coupled to larger carrier molecules, such as bovine serum
albumin or keyhole limpet hemocyanin, in order to elicit specific antibody formation.
An antibody is a Y-shaped large protein (160 kDa) bearing two combining sites
(paratopes) that bind to a limited surface (epitope) of the antigen specifically.
Epitopes are mostly located on the hydrophilic part of the antigen, and their size is
10-15 amino acid residues that can be contiguous on the protein chain (linear
epitope) or discontinuous yet spatially close on the protein chain.
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