Blood typing and cross matching test results are determined based on the reaction between the antigens and antibodies. An antigen causes the body to launch an attack against a foreign body and is known as an immune response. The attack begins when the body builds a special protein called an antibody. This antibody is uniquely designed to attack and nullify the foreign attack. In general a person's body makes antibodies only against foreign bodies and not against its own antigens.
Among the various antigens present in a person's body, the antigens found on the surface of red blood cells are important because they determine the person's blood type. When red blood cells having a certain blood type antigen are mixed with serum containing antibodies against that particular antigen, the antibodies attack and stick to the antigen. This is seen as clumping, formation of clump of cells while testing is done in a test tube.
For typing blood, a person's blood cells and serum are mixed in a test tube along with commercially prepared serum and cells. Clumping refers to the type of antigens or antibodies present and helps in determining the person's blood type. While cross matching blood, the patient's serum is mixed with cells from donor blood that may be used for transfusion. Clumping or lack of clumping in the test tube tells whether or not the blood is compatible.
There are over 600 known red blood cell antigens that are organized into 22 blood group systems. However routine blood typing and cross matching generally involves only two systems namely:
Blood typing helps in determining the blood type a person has. ABO system is used to classify the blood type. There are four main categories under the ABO system:
Blood typing also helps in determining the Rh factor of the blood. People who have the Rh protein in their blood are termed as Rh + (positive) and ones without it are termed as Rh – (negative). The Rh type is useful in deciding the blood type a person can safely receive during transfusion.
Blood type of a person is inherited and there is no normal or abnormal type. The blood type of a person is determined by checking out the following:
Type A: If the blood cells of a person agglutinate with Type A serum then the person has A blood type.
Type B: If the blood cells of a person agglutinate with Type B serum then the person has B blood type.
Type AB: If the blood cells of a person agglutinate with Type A and Type B serum then the person has AB blood type.
Type O: If the blood cell of a person does not agglutinate with Type A or Type B serum then the person has O blood type.
Understanding back typing
People with type A blood will have anti-B antibodies. People with type B blood will have anti-A antibodies. People with type O blood will have both. So if the person's:
Blood clumps only when B cells are added then the blood type of the person is A.
Blood clumps only when A cells are added then the blood type of the person is B.
Blood clumps in both cases the blood type of the person is O.
Blood does not clump when both types of blood are added then the blood type of the person is AB.
Understanding Rh results
1. If the person's blood clumps together when anti-Rh serum is added then the person is Rh+.
2. If the person's blood does not clump together when anti-Rh serum is added then the person is Rh-.
Blood typing and cross matching results
There is no normal or abnormal result. However the result may make us understand the following:
Chorionic Villus Sampling
The diagnostic procedure of taking out a sample tissue (Choroinic Villi) from the placenta to detect congenital abnormalities in a fetus is known as Chorionic Villus Sampling (CVS). With the guidance of ultrasound, the position of placenta is first determined. There are two methods - trans-cervical and trans-abdominal to perform this test. The position of the placenta helps the physician choose a suitable method. For trans-cervical CVS, parameters like the position of the uterus, the size of the gestational sac and the position of the placenta inside the uterus are first determined using abdominal ultrasound. Using a good antiseptic, the vulva, vagina and the cervix are cleansed. The abdomen is also cleansed for trans-abdominal procedure.
Trans cervical procedure: A thin plastic tube is inserted through the vagina and cervix for the trans-cervical procedure to reach the placenta. A tiny sample of chorionic villus tissue is taken out after locating the exact position of the placenta.
Trans-abdominal procedure: This procedure is similar to the earlier one, but a needle is inserted through the abdomen in this test to reach the uterus and then to the placenta. The chorionic villus sample tissue is drawn into the syringe, while the needle is guided by ultrasound.
This sample is then taken to the laboratory for evaluation. This procedure can be conducted even earlier than amniocentesis to detect any congenital defects present in the fetus. It is done at around 10 to 12 weeks after the last menstruation. Study of the DNA, chromosomes and enzymes of the fetus can be conducted using the sample taken out during the test. Results are available within a week or two. If there are any abnormalities found in the fetus, it is easy to conduct a therapeutic abortion, in case it is necessary. Pregnant women over the age of 35 who are at risk for giving birth to a baby with Downs Syndrome or those who have had birth defects in an earlier pregnancy are advised this test. For detecting neural tube defects and the Rh-incompatibility, amniocentesis is a better option. Hemoglobinopathies and Tay-Sachs disease can be detected through Chorionic Villus Sampling.
The risk involved in using CVS is slightly higher when compared to amniocentesis. Some complications like rupture of the amniotic membrane, miscarriage, infection, bleeding, Rh-incompatibility in the mother if she is Rh-negative and contamination of the sample with maternal cells can occur. When CVS is performed after 10 weeks of gestational period, there is a risk for limb defects in the fetus. If the mother's blood is Rh-negative, she has to receive Rho GAM to avoid Rh incompatibility. After the CVS, it is advised to have an ultrasound done after about two or four days to ensure the fetus is fine.
Bombay Blood Group
The Bombay blood group was discovered fifty years ago in Bombay as it included varied phenotypic characteristic when compared to the standard blood typing such as A, B and O. The RH factor of the blood groups were studied to understand the antigenic structures based on which the H antigenic characteristic became very important. The phenotypic characteristic of Bombay blood group was hence discovered as Oh wherein the 'h' represents its varied nature when compared to the other blood groups.
Bombay blood group contains 'h' characteristic. The RBCs are not agglutinated by the anti-A, anti-B. Bombay Blood group is not restricted only to Indians but is noticed among Caucasians, Japanese etc. Only about 0.01% of the world population has the Bombay blood group, which makes it an essential need to maintain a record of the respective blood type individuals for emergencies.
It is very easy to assume the blood group as O if the blood cells do not agglutinate with A and B antigens. However this may lead to adverse reactions such as hemolytic transfusion rejection when the respective blood group is transfused in to the patient. Confirmatory tests are essential to identify Bombay blood group - as the protein containing H antigen clearly differs from the O blood type. Therefore confirmatory tests such as anti-H lectin Ulex Europaeus are used to detect the presence of A ,B and OH types in the RBCs but does not agglutinate the Oh type.
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Bibliography / Reference
Collection of Pages - Last revised Date: November 22, 2019