The best replacement for antigen is antibody.

The best alternatives to antigen are rated as following:
★★★★★  antibody
★★★★  pathogen
★★★  immunogen
★★★  allergen
★★  epitope
★★  hapten
★  superantigen
★  antibodies
★  forefront
★  toxin
★  antibody blood
★  allele
  mitogen
  pamp
  antibody elisa
  microbe
  agglutinogen
  antibody hepatitis b
  surface antibody
  antibody hiv
  hepatitis b surface antigen
  self antigen

Your alternative for antigen is not listed? Suggest!
Speaking about antigen, one probably shouldn't forget about adjuvant.

Other search terms related to antigen and their ratings are:
★★★★★ adjuvant
★★★★★ antibody
★★★ antibody interaction
You have in mind a companion for antigen and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of antigen in all years. Click on graph for more.
antigen
antibody
pathogen