Two kinds of specific antibodies against antigen are used, one is immobilized on the chromatographic paper and the other is
labeled with colloidal gold and infilterated into a sample pad; when the sample is added the antigen in the sample form an immunocomplex with the antibody labeled with colloidal
gold, then the complex moved and make contact with the immobilized antibody, resulting in generating a colored line
which indicated the presence of the antigen in the sample.