Blood Group

Blood Group The Objective : To give information about : 1- Types of blood group systems present on the surfaces of red blood cells ( RBCs ) . 2- Types of ABO blood group systems depending on ABO antigens present on the surfaces of RBCs . 3- The inheritance of ABO blood group system by ABO genes which located on chromosome 9 . 4- The origin of ABO antigens through ABO genes that encoded specific enzymes that added specific sugars ( antigens ) on the RBCs receptors . 5- Types of laboratory tests used for the donor and recipients blood before blood transfusions : * Bloob typing tests * Cross match .

Twenty five blood group systems have been defined on the bases of antigens located on the surfaces of red blood cells . (Figure 1) and ( Table 1 ) . Each system is a series of red cell antigens determined by either a single genetic locus or very closely linked loci . The ABO- system and RH- systems are both of key importance in determining the compatibility of blood transfusions and tissue grafts .

Figure (1)

Table (1)

ABO – System : Discovery of the ABO system by the Austrian Karl Landsteiner in 1901 marked the beginning of safe blood transfusion . There are 4 major ABO- blood types designated by the antigens present on RBCs : 1- Blood group A : Individuals have the A antigen on the surface of their RBCs , and blood serum containing Anti-B antibodies . Therefore , a group A individual can only receive blood from individuals of groups A or O ( with A being preferable ) and can donate blood to individuals of groups A or AB .

2- Blood group B : Individuals have the B antigen on their surface of their RBCs , and blood serum containing Anti-A antibodies . Therefore , a group B individual can only receive blood from individuals of groups B or O ( with B being preferable ) and can donate blood to individuals of groups B or AB . 3- Blood group AB : Individuals have both A and B antigens on the surface of their RBCs , and their blood serum does not contain any antibodies against either A or B antigen . Therefore , an individual with type AB blood can receive blood from any group ( with AB being preferable ) , but can only donate blood to another group AB individual . (Universal recipient)

4- Blood group O : Individuals do not have either A or B antigens on the surface of their RBCs , but their blood serum contains Anti- A and Anti-B antibodies . Therefore , a group O individual can only receive blood from a group O individual , but they can donate blood to individuals of any ABO blood group ( A , B , O , or AB ) . ( Universal donor ) . ( Table 2 ) .

Table 2 : Blood Transfusion Compatibilities for the ABO Blood Groups Transfusion can be given to Transfusion can be accepted from Antibodies present in serum Antigens present on RBCs Blood Group A , AB A , O Anti- B A (galactosamine A B , AB B , O Anti- A B (galactose) B AB Universal recepient A , B , AB , O None A (galactosamine) galactose Plus B AB O , A , B , AB Universal donor O Anti-A plus Anti- B

Inheritance : Blood groups are inherited from both parents . The ABO blood type is controlled by a single gene . This gene responsible for the producing of the A and B antigens and the gene is donated by the letter I . It has 3 alleles IA , IB , IO . The gene is located on chromosome 9 . The gene encodes a glycosyltransferase enzyme .

Individuals with the IA allele have the A antigen on their erythrocyte surfaces ( blood type A ) , while those with IB have the B antigen on their cell surfaces ( blood type B ) , those with both alleles express both antigens (blood type AB ) , and those with only two copies of the IO allele have neither antigen ( type O blood ) . Because the IO allele produces no antigen , Individuals who are IA IO or IB IO heterozygotes have blood types A and B respectively . ( Table3 ) .

galactocyltransferase Table 3 : Genotypes and the Corresponding Phenotypes ( Blood Group Types ) for the ABO Locus in Humans . Frequency in Population Phenotype Activity Genotype 42% A α-3-N-acetyI-D- galactosaminyltransferase IA IA, IA IO 8% B α-3-D- galactocyltransferase IB IB, IB IO 3% AB Both enzymes IA IB 47% O None IO IO

The genetics of ABO antigens were once used to rule out paternity The genetics of ABO antigens were once used to rule out paternity .as in table 4. A child inherits genes from each parent that determine his blood type . This makes blood typing useful in paternity testing . Paternity testing compares the ABO blood types of the child , mother , and alleged father.

Inheritance of Blood Types These charts show the possible blood type results for offspring. Mothers's Type Blood Type AB B A O A, B O, B O, A Fathers' Type A, B, AB O, A, B, AB Mother's Type Rh Factor Rh - Rh + Rh +, Rh - Rh +, Rh + Father's Type

The Origin of ABO Antigens : The A and B genes each code for an enzyme (glycosyl transferases ) that adds aterminal carbohydrate to RBC receptors during maturation . ( Figrue 2 ) . RBCs of type A contain an enzyme that adds N – acetyl galactosamine to the receptor ; RBCs of type B have an enzyme that adds D – galactose ; RBCs of type AB contain both enzymes that add both carbohydrates ; and RBCs of type O lack the genes and enzymes to add a terminal molecule . ( Figure 3 ) . Then the ABO antigens are not primery gene products but instead they are the enzymatic reaction products of enzymes called glycosyltransferases which is encoded by the ABO gene .

Figure (2) Figure (3)

The Origin of ABO Antigens

Codominance : When both alleles of pair are fully expressed in a heterozygote , they are called Codominants . In humans , the ABO blood group antigens are a good example . Amating between a homozygous A- type person ( IA IA ) and a homozygous B- type person ( IB IB ) would result in all heterozygous AB- type ( IA IB ) offspring . ( Figure 4 )

Figure (4)

Mating between heterozygoutes ( IA IB x IA IB ) would result in a ratio of : 1 A- type ( IA IA ) : 2 AB- type ( IA IB ) : 1 B- type ( IB IB). Aphenotypic ratio of 1 : 2 : 1 has thus replaced the 3 : 1 ratio , because the alleles are codominant . The functional alleles A and B provide activities that are codominant with one another and dominant over O group .

The Source of these ( anti A ) and ( anti B ) antibodies: It appears that they develop in early infancy due to exposure to certain heterophile antigens that are widely distributed in nature . These antigens are surface molecules on bacteria and plant cells that mimic the surface of A and B antigens . Exposure to these sources stimulates the production of corresponding antibodies (IgM). (Antibodies are proteins produced by the immune system that comnine with specific antigens ; Hence , anti A combines with antigen A) . Because of their specificity for the corresponding antigens these antibodies are used in standard tests to determine blood type .

Blood Transfusions : A number of laboratory tests must be completed before blood can be transfused : 1- Blood Typing : The individual blood types of donor and recepient must be determined .Using a standard technique , drops of blood are mixed with antisera that contain antibodies against the A and B antigens , and are then observed for the evidence of agglutination . ( Figure 5 ) . 2- Screening for possible infectious agents that could be transmitted by blood transfusion such as : Human Immunodeficiency Virus ( HIV ) 1 and 2 , Hepatitis B Virus (HBV) , Hepatitis C Virus (HCV) and some bactria and parasites . 3- Crossmatch ( Compatibility Test ) : The general rule of compatibility is that the RBC antigens of the donor must not be agglutinated by antibodies in recepient’s blood .

Figure (5)

For example : If the donor is A type and the recepient is B type For example : If the donor is A type and the recepient is B type .The RBCs of the type A donor contain antigen A , while the serum of the type B recepient contains anti-A antibodies that can agglutinate donor RBCs (agglutination : aggregation by antibodies of RBCs into clumps that settle Agglutinated RBCs can clog blood vessels and stop circulation in vital organs . And the activation of complement by antibodies on the RBCs can cause hemolysis and anemia , fever , jaundice . (Transfusion reaction :occurs When incompatibile blood is transfused , specifically if antibodies in the recipient’s serum cause rapid RBC destruction in the proposed donor ). ( Figure 6 ) .

Figure (6)

The ideal practice is to transfuse blood that is a perfect match ( A to A , B to B ) . But even in this event blood samples must be cross matched prior to transfusion because other blood group incompatibilities can exist . The primary purpose of the major cross match or compatibility test , is to prevent a possible transfusion reaction . The aim of cross matching is to ensure that the blood of a recepient does not contain antibodies that will be able to react with and destroy transfused ( donor ) RBCs . To begin the crossmatch , blood from adonor with the same ABO and Rh type as the recipient is selected . In atest tube , serum from the patient is mixed with RBCs from the donor . If clumping occurs , the blood is not compatibile . If clumping does not occur the blood is compatibile . ( Figure 7 ) .

In an emergency , when there is not enough time for blood typing and crossmatching O red blood cells may be given . preferably Rh- negative . O- blood type is called the Universal donor because it has no ABO antigens for a patient’s antibodies to attack . In contrast , AB+ blood type is called the Universal recipient because it has no ABO antibodies to attack the antigens on transfused red blood cells . If there is time for blood typing , RBCs of the recipient type ( type specific cells ) are given . In either case , the crossmatch is continued , even though the transfusion has begun .

Blood donors and blood recepients must have compatible blood types . ( Table 4 : Blood compatibility chart ) illustrates how people with different blood types can receive or donate blood . An A- person , for example , can receive either O- or A- , and can donate to people with AB+ , AB- , A+ or A- blood . An O-person can donate blood to people with any type , and is termed a Universal donor . An AB+person can receive blood of any type , and is termed Universal recepient .

Table (4)

Figure (7)

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