ectoantigen

Any toxin or other excitor of antibody formation, separate or separable from its source. SYN: exoantigen.

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ec·to·an·ti·gen (ek″to-anґtĭ-jən) an antigen which seems to be loosely attached to the outside of bacteria so that it can be readily removed by shaking them in physiologic sodium chloride solution; also an antigen formed in the ectoplasm of a bacterium.