Using photogenerated glycan arrays, we characterized a large panel of synthetic carbohydrates for their antigenic reactivities with pathogen-specific antibodies. We discovered that rabbit IgG antibodies elicited by Bacillus anthracis spores specifically recognize a tetrasaccharide chain that decorates the outermost surfaces of the B. anthracis exosporium. Since this sugar moiety is highly specific for the spores of B. anthracis, it appears to be a key biomarker for detection of B. anthracis spores and development of novel vaccines that target anthrax spores.