Structures of the immunodominant protein P46 from Mycoplasma hyopneumoniae have been determined by X-ray crystallography and it is shown that P46 can bind a diversity of oligosaccharides, particularly xylose, which exhibits a very high affinity for this protein. Structures of a monoclonal antibody that was raised against M. hyopnemoniae cells and specifically recognizes P46 are also reported, both alone and in complex with P46.