What is the Rh Factor in Blood - ABO Blood Group System

Blood is classified into groups according to the presence of particular antigens, or proteins, on the blood cells.
A knowledge of a person's blood group is important when a blood transfusion is necessary in tissue and organ transplants, in cases of disputed paternity, and in preventing problems with newborn children.
The two most important classification systems are the ABO system and the Rhesus (Rh) system.

ABO blood group system

The ABO system distinguishes four blood groups: A, B, AB, and O. All blood can be classified in one of these groups.
A and B represent proteins that are found on the surface of blood cells. O represents the absence of either of these proteins.
Since a person receives two genes for blood groups (one from each parent), one can fall into four groups: A, representing either two A genes (AA) or one A gene and an O gene for which there is no protein produced; B, similar to A; O, if one receives O genes from both the parents; or AB.
People develop antibodies that fight against the proteins they do not possess.
For example, someone with type A would have antibodies against type B blood. If someone is given a transfusion of blood against which he or she has antibodies, a serious reaction can occur.
Since a person with AB blood has both groups, he or she may receive A, B, O, or AB blood. Conversely, since a person with O blood has neither protein, he or she cannot receive A, B, or AB blood, but O blood can be given to people with any ABO type.

What is the Rh factor in blood?

The Rhesus system defines two blood types: blood in which the Rh factor is present (Rhesus positive, or Rh + ), and that in which it is not (Rhesus negative, or Rh -).
In Western countries, about 85 percent of people are Rh +.

The Rhesus system and the ABO bear no correlation whatsoever with each other, but for the sake of clarity the ABO and Rhesus classifications are generally combined when stating a blood group (for example, A + , 0-, etc.).