Human Blood: ABO Blood Types
(1868-1943)
The most well-known   and medically important blood types are in the ABO
group.  They were discovered in 1900 and 1901 at the University of Vienna   by Karl Landsteiner in the process of trying to learn why blood transfusions   sometimes cause death and at other times save a patient.  In 1930, he   belatedly received the Nobel Prize for his discovery of blood types.
All humans and many other primates can be typed for   the ABO blood group.  There are four principal types: A, B,   AB, and O.  There are two antigens and two antibodies that are mostly responsible for the ABO   types.  The specific combination of these   four components determines an individual's  type in most cases.  The table below shows    the possible permutations of antigens and antibodies with the corresponding ABO type ("yes" indicates the presence of a component and "no" indicates its absence in the    blood of an individual).
 
 
 
It is easy and   inexpensive to determine an individual's ABO type from a few drops of blood.  A serum   containing anti-A    antibodies is mixed with some of the   blood.  Another serum with anti-B    antibodies is mixed with   the remaining sample.   Whether or not agglutination occurs in either sample   indicates the ABO type.   It is a simple process of elimination of the possibilities.  For instance, if an individual's blood sample is agglutinated by the anti-A    antibody, but not the     anti-B antibody, it means   that the A    antigen is present but not the B    antigen.  Therefore, the blood type is A.
 
Genetic Inheritance   Patterns
Research carried out in Heidelberg,    Germany by Ludwik Hirszfeld and Emil von Dungern in 1910 and 1911 showed that    the ABO blood types are inherited.  We now know that they are   determined by genes on chromosome 9, and they do not change as a result of environmental influences during life.  An   individual's ABO type results from the inheritance of 1 of 3 alleles (A,   B, or   O) from each parent.  The possible   outcomes are shown below:
The possible ABO alleles for one
parent are in the top row and the
alleles of the         other are in the left
column.  Offspring genotypes
OO
(O)
Both   A and   B alleles are dominant over   O.  As a result, individuals who have   an AO genotype will have an A phenotype .  People who are type O have   OO   genotypes.  In other words, they inherited a recessive O allele from both parents. The   A and   B alleles   are codominant.  Therefore, if an A is inherited from one parent and a   B from the other, the phenotype will be   AB.   Agglutination tests will show that these individuals have the   characteristics of both type A   and type B blood.
CAUTION: the inheritance of ABO blood types does not always follow such straightforward rules of   inheritance.  If you wish to explore the reason why this is true, select the Bombay Phenotype   button below.
   Bombay Phenotype    
ABO Blood    type antigens are not only found on the surface of red cells.  They are    also normally secreted by some people in their body fluids, including saliva,   tears, and urine.  Whether someone is able to secrete them is genetically controlled.  Police agencies now routinely use this so-called secretor system data   to identify potential victims and criminals when blood samples are not   available.
Despite   the fact that the blood types of children are solely determined by inheritance   from their parents, paternity in the U.S. and many other nations can no longer be legally established based on   conventional blood typing.  To do that, it is necessary to compare HLA types and/or DNA sequences.  The use of    DNA is more accurate in determining paternity, but it is also more expensive    than HLA typing.
Antibodies   to alien antigens in the ABO group are usually present in our plasma prior to the   first contact with blood of a different ABO type.  This may be partly   explained by the fact that these antigens are also produced by certain   bacteria and possibly some plants.  When we come in contact with them, our bodies may develop long-term    active immunity   to their antigens and subsequently to the same antigens on the surface of red   blood cells.  This usually occurs in babies within the first six months   following their birth.
Environmental Factors
While blood types are   100% genetically inherited, the environment potentially can determine which blood types   in a population will be passed on more frequently to the next generation.  It does this through natural selection .  Specific ABO blood types are thought to be linked with increased or decreased susceptibility   to particular diseases.  For instance, individuals with type A blood are   at   a somewhat higher risk of contracting smallpox and developing cancer of the   esophagus, pancreas, and stomach.  People who are type O are at a   higher risk for contracting cholera and plague as well as developing   duodenal and peptic ulcers.  Research suggests that they are also more   tasty to mosquitoes.  That could be a significant factor in contracting   malaria.
 
NOTE:  A small number of people have two    different ABO blood types.  They are not simply AB codominant.  Apparently, most of these  blood chimera
individuals shared a blood supply    with their non-identical twin before birth.  In some cases, people are    unaware that they had a twin because he or she died early in gestation and was    spontaneously aborted.  As many as 8% of non-identical twins may have chimeric blood.  Some people are microchimeric--they have a small amount    of blood of a different type in their system that has persisted from a blood    transfusion or passed across the placental barrier from their mother before    birth.  Likewise, fetal blood can pass into a mother's system.  This    fact has led some researchers to suggest that the significantly higher    frequency of autoimmune disorders in women is a result of the presence of    foreign white blood cells that had come from their unborn children during pregnancy.
NEWS:  An international team of researchers led by Henrick Clausen of the   University of Copenhagen, Denmark have discovered a bacterial enzyme that   can convert red blood cells of types A, B, and AB into O by stripping away   their identifying surface antigens.  This has the potential for   dramatically improving the safety of blood transfusions.  Clinical   trials of this technique are now underway.  ("Bacterial   Glycosidases for the Production of Universal Red Blood Cells", published   online in Nature Biotechnology,   April 1, 2007.)
NEWS:  A research team   led by Peer Bork of the European Molecular Biology Laboratory in Heidleberg,   Germany discovered that people can be classified into one of 3 distinct   types based on the kinds of bacteria in their guts.  They refer to them   as enterotypes.  This type system is independent of blood types and may   have equally important implications for peoples' health.  How enterotypes are established   is not known, but the authors suggest that babies may be randomly colonized   by different species of bacteria and that they alter the gut so that only   certain species of bacteria can live there.  ("Enterotypes of the Human   Gut Microbiome", published online in Nature, April 20, 2011.)