I don't know where you read that, but no microbial antigens are even slightly similar to human antigens, given that we are separated by at least 1 billion years of evolution. The fact that someone with blood group 'A' rejects blood from someone from group 'B' is simply a testament to the extraordinary versatility of our immune system. Although the differences in the glycoproteins indicating our blood types are subtle, they are sufficiently different that our immune system can recognize them as 'foreign.'
As far as microbial antigens go (and this is something I work with on a daily basis), they are sufficiently different that, in many cases, we do not need the adaptive immune response (eg, antibodies and B cells), but simply the innate immune response (macrophages and dendritic cells) to respond to foreign antigen (eg, lipopolysaccharide and CpG methylated DNA). Given that microbial physiology is so different from our own, it is (almost) impossible that antibodies against blood groups are initiated by microbes.