Microbial surface antigens that frequently exist in the form of filamentous projections (pili or fimbriae) and bind to specific receptors on epithelial cell membranes; usually classified according to their ability to induce agglutination of erythrocytes from various species, their differential attachment to epithelial cells of various origins, or their susceptibility to reversal of such binding activities in the presence of mannose.
[L. ad-haereo, pp. ad-haesum, to stick to, + -in]
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Examples: glitazone, GI cocktail, etc.