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In this article, Heidelberger reviewed data on cross-reactions between 46 of the over 80 pneumococcal types identified by then, with particular focus on determining why some of the cross-reactions were reciprocal (meaning that antibodies formed against two or more different pneumococcal polysaccharide antigens reacted with the other antigens in this group as well) while others were only unilateral (meaning that while an antibody formed against one pneumococcal antigen reacted with another such antigen as well, the latter's antibodies did not react with the former antigen).