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.