Medicine defines four different blood types, known as A, O, B and AB. Within these definitions there is a positive and negative form of each, thus a person can be referred to as “A positive” or “O negative”. A person’s blood type letter is defined by the antigens (a kind of substance which produces antibodies) they have in their blood and this is a trait we inherit from our parents. A person with “A” antigens has “A” blood, and likewise having “B” antigens means you have group B blood. If you have both kinds of antigen, your blood group will be defined as AB. If you have no antigens at all, you have O-type blood.
The forms of blood do not all agree with one another, meaning that it is crucial to know someone’s blood type before giving them a transfusion. A person with B-type blood will react against the introduction of A-type blood as it will perceive the antigens in that blood as a threat and attack them; the same will work in reverse. The exception to this is type O, which can be given to any of the other groups as it has no antigens which would provoke an attack.
Put simply, if you have A-type blood you can receive blood from types A and O. B-type blood can accept blood from types B and O. If you have AB type blood you can receive blood from A, B, AB and O groups; however if you have O blood, the absence of antigens means that you can only receive blood from other O-types.
A second factor is the presence (or not) of another antigen, called Rh (Rhesus). If you have this antigen (most, but not all, people have it- most figures suggest about 80 to 90 per cent of people do), your blood type is referred to as “positive”, and if you don’t have it, your blood type will be called “negative’. Thus someone who has A-type blood and who also has the rhesus antigen is known as A positive, whereas someone who has O blood and does not have the antigen is O negative. In the same way that blood group A won’t mix with blood group B, a positive and a negative must not be mixed together.
An interesting footnote about blood groups: In Japan, there is still a strong belief that a person’s blood group indicates numerous things about that individual;, to the extent that it looks like astrology. Almost every Japanese knows his or her own blood type, and discussion of a celebrity of public figure will often include that person’s blood group as a topic for discussion. Although the medical community at large refutes this theory as unscientific, if not somewhat racist, it remains popular in Japan and South Korea as well.
Sources
http://chapters.redcross.org/br/northernohio/INFO/bloodtype.html
http://www.blood.co.uk/pages/e13basic.html
http://en.wikipedia.org/wiki/ABO_blood_group_system
http://en.wikipedia.org/wiki/Blood_types_in_Japanese_culture
