It remains to be seen whether this appearance is related to inborn differences between individuals or it is the result of some damage of bacterial kind. This was the first evidence that blood variations exist in humans – it was believed that all humans have similar blood. The next year, in 1901, he made a definitive observation that blood serum of an individual would agglutinate with only those of certain individuals. Based on this he classified human blood into three groups, namely group A, group B, and group C. He defined that group A blood agglutinates with group B, but never with its own type. Similarly, group B blood agglutinates with group A. Group C blood is different in that it agglutinates with both A and B.This was the discovery of blood groups for which Landsteiner was awarded the Nobel Prize in Physiology or Medicine in 1930. In his paper, he referred to the specific blood group interactions as isoagglutination, and also introduced the concept of agglutinins (antibodies), which is the actual basis of antigen-antibody reaction in the ABO system. He asserted:

[It] may be said that there exist at least two different types of agglutinins, one in A, another one in B, and both together in C. The red blood cells are inert to the agglutinins which are present in the same serum. Thus, he discovered two antigens (agglutinogens A and B) and two antibodies (agglutinins – anti-A and anti-B).
Several studies have observed that a relative down-regulation of GTA and GTB occurs in oral carcinomas in association with tumor development. More recently, a genome wide association study (GWAS) has identified variants in the ABO locus associated with susceptibility to pancreatic cancer. In addition, another large GWAS study has associated ABO-histo blood groups as well as FUT2 secretor status with the presence in the intestinal microbiome of specific bacterial species. In this case the association was with Bacteroides and Faecalibacterium spp. Bacteroides of the same OTU (operational taxonomic unit) have been shown to be associated with inflammatory bowel disease, thus the study suggests an important role for the ABO histo-blood group antigens as candidates for direct modulation of the human microbiome in health and disease. Clinical marker
A multi-locus genetic risk score study based on a combination of 27 loci, including the ABO gene, identified individuals at increased risk for both incident and recurrent coronary artery disease events, as well as an enhanced clinical benefit from statin therapy. The study was based on a community cohort study (the Malmo Diet and Cancer study) and four additional randomized controlled trials of primary prevention cohorts (JUPITER and ASCOT) and secondary prevention cohorts (CARE and PROVE IT-TIMI 22). Alteration of ABO antigens for transfusion
In April 2007, an international team of researchers announced in the journal Nature Biotechnology an inexpensive and efficient way to convert types A, B, and AB blood into type O.