Within the ABO/Rhesus Group systems which are used to classify most blood types, there are several rare blood types. The rarest is AB-, with less than one percent of the world's population having this blood type. B- and O- are also very rare, each accounting for less than 5% of the world's population. However, there are over 30 recognized blood typing systems beyond these basic two, creating a plethora of rare blood types, some of which appear only in a handful of people.

Blood type is determined by the presence of certain antigens in the blood. The A and B antigens are very common, making it easy to classify people by which antigen they have, while people with Type O blood have neither antigen. The negative or positive symbol after the blood type indicates the presence or lack thereof of the Rhesus Factor. However, other antigens beyond A and B can be present, and these antigens can react with blood from certain donors. Someone with a rare blood type might appear, for example, to be A+, but he or she might lack another antigen, which could cause a bad reaction with A+ donor blood which contained that antigen.

People in the Bombay Blood Group lack A and B antigens, which would normally place them in the O grouping, except that they lack H antigen, a substance present in people with Type O blood. This means that if they are infused with blood from an O donor, they can get sick. This blood type is also referred to as the “hh Blood Group,” and it is named for the region of India where it was discovered.

Some of the other antigens used in blood typing include: D Factor, C Factor, E Factor, M Factor, S Factor, Le(a) Factor, K Factor, Fy(a) Factor, Jk(b) Factor, and Fy(b) Factor, among others. This means that someone could have a blood type such as AB-: Fy(b)-, K-. If blood from an AB- negative donor which contained the K Factor was transfused, this person could experience a reaction. Sometimes, these factors are referred to with names, as in “Duffy Negative” for people who lack Fy(a) and Fy(b), referencing a specific patient who first exhibited the trait.

Blood type is an inherited trait, and many rare blood types are found in specific communities and ethnic groups. African-Americans, for example, are more at risk of being Duffy Negative. This is why it is important for people from all ethnic groups to donate blood, to increase the probability of finding a matching donor. When blood from more than 200 donors must be screened to find a match for a patient, that patient has a blood type which is considered rare, but some people with rare blood types would be lucky to find blood which matched theirs in one of 200 donations. Some people with rare blood types bank their own blood in advance of surgical procedures to ensure that blood is available to them.