FIG. 1.

Proposed structures of the Aspergillus fumigatus galactomannan (adapted from references 8 and 9 with permission) and the Cryptococcus neoformans galactoxylomannan (adapted from reference 1 with permission). The repeating unit of the galactomannan secreted by Aspergillus fumigatus consists of a linear core of mannan residues with α-1,2- and α-1,6-linked residues. Antigenic side chains composed exclusively of β-1,5 galactofuranosyl units are linked to the C-6 and C-3 positions of the α-1,2-linked mannose units of the core. These side chains have an average degree of polymerization of 4 and are the target of the antigalactomannan monoclonal antibody employed in the Platelia Aspergillus test. The repeating unit of the Cryptococcus neoformans galactoxylomannan consists of a backbone of α-1,6-linked galactose units, with side chains composed of galactose, mannose, xylose, and O-acetyl residues branched on alternate galactose residues. Small quantities of galactofuranose are present in the GalXM in some, but not all, C. neoformans isolates (6). The linkage of these galactofuranose units has not been characterized (14). M, mannan; G, galactose; Gf, galactofuranose; X, xylose.