What is a Blood Type?
During a type and screen, a sample of blood is collected from the person and sent to a laboratory for analysis. The blood is first tested to determine the person's blood type, which is based on the presence or absence of certain antigens on the surface of the red blood cells. There are four major blood types: A, B, AB, and O.
Additionally, The Rh factor is a protein that is found on the surface of red blood cells. Specifically, the Rh factor refers to a group of antigens known as the Rhesus antigens, with the most important of these antigens being the D antigen. People who have the D antigen are considered Rh positive (Rh+), while those who do not have the D antigen are considered Rh negative (Rh-).
The D antigen is determined by an individual's genetics. The gene that controls the D antigen is located on chromosome 1. If an individual inherits the dominant D gene from one or both parents, they will be Rh positive. If they inherit two recessive d genes, they will be Rh negative.
Most people (about 85%) are Rh positive, while the remaining 15% are Rh negative. Rh factor is important in pregnancy because if a woman who is Rh negative becomes pregnant with a fetus who is Rh positive, there is a risk that the woman's immune system may produce antibodies against the Rh factor of the fetus, which can lead to complications in future pregnancies. To prevent this, Rh-negative women who become pregnant with an Rh-positive fetus may be given a medication called Rh immune globulin (RhoGAM). RhoGAM works by blocking the woman's immune system from producing antibodies against the D antigen of the fetus.
What is an Antibody Screen?
The blood is also screened for antibodies that may react with transfused blood. Alloantibodies are antibodies produced by an individual's immune system in response to exposure to foreign antigens from transfused blood. In other words, alloantibodies are antibodies that are directed against antigens that are not present on a person's own red blood cells. These antibodies can develop when a person is exposed to red blood cells that have different blood group antigens than their own.
Alloantibodies
Alloantibodies can cause serious complications during blood transfusions. When a person receives a transfusion of blood that contains foreign antigens, their immune system can produce alloantibodies against those antigens. If the person receives another transfusion with incompatible blood in the future, their immune system can recognize the foreign antigens and initiate an immune response, resulting in a transfusion reaction. The A and B antigens are the most commonly known antigens on a persons Red Blood Cells that are used for compatibility. This is where we get the blood types A, B, O, and AB. However there are DOZENS more known antigen systems on Red Blood Cells. It is not routine for labs to screen for these antigens on a patient due to time and cost, but if they create an alloantibody they will then need tested for these antigens that they create an antibody towards.
Alloantibodies may also be made during the stages of pregnancy. Alloimmunization in pregnancy occurs when a woman's immune system produces antibodies against foreign blood group antigens present in the blood of her developing fetus. This can occur when the mother has a different blood type from the fetus, and the fetus inherits blood group antigens from the father that are foreign to the mother.
If the mother is Rh-negative and the fetus is Rh-positive, for example, the mother's immune system may produce Rh antibodies that can cross the placenta and attack the fetal red blood cells. This can result in hemolytic disease of the newborn (HDN), a condition in which the fetus or newborn experiences the destruction of red blood cells and can lead to anemia, jaundice, and other serious complications.
Alloantibodies are often detected through a blood test called an antibody screen, which is performed prior to a blood transfusion to ensure that the donor blood is compatible with the recipient's blood type. If alloantibodies are detected, the transfusion process must be carefully managed to avoid a transfusion reaction. In some cases, it may be necessary to use blood from a special donor source or to perform additional testing to identify compatible blood products.
What is a Crossmatch?
You may hear the term Type and Cross, which is a bit of a misnomer. A crossmatch is a test performed using the Type and Screen specimen. A crossmatch involves mixing a sample of the recipient's blood serum with a sample of the donor's red blood cells and observing whether there is agglutination (clumping) of the cells. If the recipient's antibodies react with the donor's cells, this indicates that the blood is incompatible and a transfusion could result in a severe transfusion reaction.
There are two main types of crossmatches: the immediate spin crossmatch and the full crossmatch. The immediate spin crossmatch involves mixing the donor's cells with the recipient's serum and observing for agglutination at room temperature. The full crossmatch is a more comprehensive test that involves incubating the donor's cells with the recipient's serum at 37°C to mimic the conditions inside the body and observing for agglutination.
Patient's with a proven history of a negative antibody screen may be eligible for an "Electronic Crossmatch" in which the laboratory information system scans the unit of blood through the patient's history and agrees that the unit is "electronically" compatible based on the correct choice of Blood Type and negative antibody history. This saves time and efficiency for the lab worker and the patient as no physical testing needs to be performed. Patient's with blood typing issues or antibodies are not eligible for electronic crossmatch.
The crossmatch is a critical step in ensuring safe blood transfusion and is performed for every blood product that is transfused to a patient. If an incompatible crossmatch is detected, the transfusion must be halted immediately, and alternative compatible blood products must be obtained.