Define Antigen and Antibody. Name any two diagnostic kits based upon them.

(N/A) An antigen is a foreign substance that elicits the formation of an antibody. An antibody is a protein that is synthesized by the immune system in response to the presence of an antigen. Antigen and antibody show a high degree of specificity in binding to each other. Two diagnostic kits based on antigen-antibody interaction are:
$(a)$ $ELISA$ (Enzyme-Linked Immunosorbent Assay) for the detection of $HIV$.
$(b)$ Pregnancy test kits (which detect $hCG$ hormone).