Similarity profiles between helper T cell epitopes (of self or microbial antigens and allergens) and human or microbial SWISSPROT collections were produced. For each antigen, both collections yielded largely overlapping profiles, demonstrating that self-nonself discrimination does not rely on qualitative features that distinguish human from microbial peptides. … Epitopes (on self and nonself antigens) can cross-stimulate T cells at increasing potency as their similarity with nonself augments.