any antigen-bearing foreign material that enters the body and normally stimulates an attack by the body's immune system (distinguished from self).
Origin of nonself
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
That which the immune system identifies as foreign to the body.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.