ANTIGENS AND ANTIBODIES
Definition:
- Blood group system
- A series of antigens exhibiting similar serological and physiological characteristics, and inherited according to a specific pattern.
Importance of the ABO system:
Most important (clinically significant) Blood Group System for transfusion practice
Why?
This is the only blood group system in which antibodies are consistently, predictably, and naturally present in the serum of people who lack the antigen. Therefore ABO compatibility between donor and recipient is crucial since these strong, naturally occurring A and B antibodies are IgM and can readily activate complement and cause agglutination. If ABO antibodies react with antigens in vivo, result is acute hemolysis and possibly death.
Indications for ABO grouping:
ABO grouping is required for all of the following individuals:
- Blood Donors-since it can be life threatening to give the wrong ABO group to the patient.
- Transfusion recipients-since we need to know the donor blood is ABO compatible.
- Transplant Candidates and Donors-ABO antigens are found in other tissues as well. Therefore the transplant candidates and donors must be compatible.
- Prenatal Patients-To determine whether the mothers may have babies who are suffering from ABO-HDN. It is also beneficial to know the ABO group should she start hemorrhaging.
- Newborns (sometimes) If the baby is demonstrating symptoms of Hemolytic Disease of the Newborn, the ABO group needs to be determined along with Rh and others.
- Paternity testing Since the inheritance of the ABO Blood Group System is very specific, this serves as one of the first methods to determine the likelihood that the accused father is the father or not.
Discovery of the ABO system:
In 1900 Karl Landsteiner reported a series of tests, which identified the ABO Blood Group System. In 1910 he won Nobel prize for medicine for this discovery. He mixed the serum and cells of all the researchers in his lab and found four different patterns of agglutination. From those studies he developed what we now know as Landsteiner's rules for the ABO Blood Group:
- A person does not have antibody to his own antigens
- Each person has antibody to the antigen he lacks (only in the ABO system)
- Below are the four blood groups and the antigens and the expected, naturally-occurring antibodies present.
| BLOOD GROUP | ANTIGEN | ANTIBODY |
| A | A | anti-B |
| B | B | anti-A |
| AB | A and B | neither |
| O | neither anti-A or anti-B | anti-A,B |
Incidence (%) of ABO Blood Groups in the US Population | ||
| ABO Group | Whites | Blacks |
| O | 45 | 49 |
| A | 40 | 27 |
| B | 11 | 20 |
| AB | 4 | 4 |
ABO Typing
ABO typing involves both antigen typing and antibody detection. The antigen typing is referred to as the forward typing and the antibody detection is the reverse typing
- The forward typing determines antigens on patient's or donor's cells
a. Cells are tested with the antisera reagents anti-A, anti-B, (and in the case of donor cells anti-A,B)
b. Reagents are either made from hyperimmunized human sources, or monoclonal antibodies.
c. One advantages of the monoclonal antibodies are the antibody strength.
d. Another advantage of monoclonals: human source reagents can transmit infectious disease (hepatitis). - Reverse typing determines antibodies in patient's or donor's serum or plasma
a. Serum tested with reagent A1 cells and B cells
b. Reverse grouping is also known as backtyping or serum confirmation
Routine ABO Typing | |||||
Reaction of Cells Tested With | Red Cell ABO Group | Reaction of Serum Tested Against | Reverse ABO Group | ||
| Anti-A | Anti-B | A1 Cells | B Cells | ||
| 0 | 0 | O | + | + | O |
| + | 0 | A | 0 | + | A |
| 0 | + | B | + | 0 | B |
| + | + | AB | 0 | 0 | AB |
Discrepancies in ABO typing
- Results of forward and reverse typing must agree before reporting out blood type as seen in the about table.
- If forward and reverse do not agree, must identify cause of discrepancy.
- If cannot resolve discrepancy, must report out blood type as UNKNOWN and give group O blood
Characteristics of ABO antigens:
ABO antigens are glycolipid in nature, meaning they are oligosaccharides attached directly to lipids on red cell membrane. These antigens stick out from red cell membrane and there are many antigen sites per red blood cell (approximately 800,000)
Besides their presence on red blood cells, soluble antigens can be present in plasma, saliva, and other secretions. These antigens are also expressed on tissues other than red cells. This last fact is important to consider in organ transplantation.
ABO antigens are only moderately well developed at birth. Therefore ABO-HDN not as severe as other kinds of Hemolytic Disease of the Newborn. .
Characteristics of ABO antibodies:
- These are expected naturally occurring antibodies that occur without exposure to red cells containing the antigen. (There is some evidence that similar antigens found in certain bacteria, like E.coli, stimulate antibody production in individuals who lack the specific A and B antigens.)
- Immunoglobulin M antibodies, predominantly
- They react in saline and readily agglutinate. Due to the position of the antigen and the IgM antibodies it is not necessary to overcome the zeta potential.
- Their optimum temperature is less than 30oC, but reactions do take place at body temperature
- Not only are these antibodies expected and naturally occurring, they are also commonly present in high titer, 1/128 or 1/256.
- They are absent at birth and start to appear around 3-6 months as result of stimulus by bacterial polysaccharides. (For this reason, newborn blood is only forward typed.)
ABO INHERITANCE
Inheritance Terminology:
- gene:
- determines specific inherited trait (ex. blood type)
- chromosome:
- unit of inheritance. Carries genes. 23 pairs of chromosomes per person, carrying many genes. One chromosome inherited from mother, one from father
- locus:
- site on chromosome where specific gene is located
- allele:
- alternate choice of genes at a locus (ex. A or B; C or c, Lewis a or Lewis b)
- homozygous:
- alleles are the same for any given trait on both chromosome (ex. A/A)
- heterozygous:
- alleles for a given trait are different on each chromosome (ex. A/B or A/O)
- phenotype:
- observed inherited trait (ex. group A or Rh positive)
- genotype:
- actual genetic information for a trait carried on each chromosome (ex. O/O or A/O)
- dominant:
- the expressed characteristic on one chromosome takes precedence over the characteristic determined on the other chromosome (ex. A/O types as A)
- co-dominant:
- the characteristics determined by the genes on both chromosomes are both expressed - neither is dominant over the other (ex. A/B types as AB)
- recessive:
- the characteristic determined by the allele will only be expressed if the same allele is on the other chromosome also (ex. can type as O only when genotype is O/O)
ABO Genes
The A and B genes found on chromosome #9. We inherit one gene (allele) from our father and one from our mother. The two co-dominant alleles are A or B. Anytime an individual inherits an A or B gene it will be expressed.
The O gene signifies lack of A or B antigens. It is not expressed unless this gene is inherited from both parents (OO). Therefore the O gene is recessive.
Below is the example of two individuals who are A. One inherited only one A gene along with an O gene and is therefore heterozygous. The other inherited 2 A genes and is homozygous for A.
 | |
1 = A/A
|
2 = A/O
|
| 1 = Homozygous A | 2 = Heterozygous A |
| Phenotype A | Phenotype A |
| Genotype A/A | Genotype A/0 |
| Can Contribute Only an A Gene to Offspring | Can Contribute A or O Gene to Offspring |
Inheritance Patterns
We can't determine genotypes of A or B people unless family studies are done. Some basic rules of ABO inheritance are as follows:
- A/A parent can only pass along A gene
- A/O parent can pass along either A or O gene
- B/B parent can only pass along B gene
- B/O parent can pass along either B or O gene
- O/O parent can only pass along O gene
- AB parent can pass along either A or B gene
ABO phenotypes and genotypes
1. Group A phenotype = A/A or A/O genotype
2. Group B phenotype = B/B or B/O genotype
3. Group O phenotype = O/O genotype
4. Group AB phenotype = A/B genotype
Offspring possibilities
Possibilities of an A/O mating with a B/O: (Children's genotypes in purple)
| Mother's Genes | Father's Genes | |
| B | O | |
| A | AB | AO |
| O | BO | OO |
Possibilities of AA mating with BB: (Children's genotypes in purple)
| Mother's Genes | Father's Genes | |
| B | B | |
| A | AB | AB |
| A | AB | AB |
Possibilities of an A/A mating with a B/O: (Children's genotypes in purple)
Mother's Genes
|
Father's Genes
| |
| B | O | |
| A | AB | AO |
| A | AB | AO |
Possibilities of an A/A mating with an O/O:
| Mother's Genes | Father's Genes | |
| O | O | |
| A | AO | AO |
| A | AO | AO |
Possibilities of an A/O mating with an O/O:
Mother's Genes
|
Father's Genes
| |
| O | O | |
| A | AO | AO |
| O | OO | OO |
Possibilities of an A/B mating with a O/O:
Mother's Genes
|
Father's Genes
| |
| O | O | |
| A | AO | AO |
| B | BO | BO |
BIOCHEMISTRY OF THE ABO SYSTEM
The ABO antigens are terminal sugars found at the end of long sugar chains (oligosaccharides) that are attached to lipids on the red cell membrane. The A and B antigens are the last sugar added to the chain. The "O" antigen is the lack of A or B antigens but it does have the most amount of next to last terminal sugar that is called the H antigen.
Production of A, B, and H antigens
The production of A, B and H antigens are controlled by the action of transferases. These transferases are enzymes that catalyze (or control) addition of specific sugars to the oligosaccharide chain. The H, A, or B genes each produce a different transferase, which adds a different specific sugar to the oligosaccharide chain.
To understand the process let's look at the sequence of events:
- Precursor chain of sugars is formed most frequently as either Type 1 or Type 2 depending on the linkage site between the N-acetylglucosamine (G1cNAc) and Galactose (Gal).
- H gene causes L-fucose to be added to the terminal sugar of precursor chain, producing H antigen (shown in this diagram of a Type 2 H antigen saccharide chaine).
- Either A gene causes N-acetyl-galactosamine to be added to H substance, producing A antigen, (shown in this diagram) or
- B gene causes D-galactose to be added to H substance, producing B antigen.
- If both A and B genes present, some H-chains converted to A antigen, some converted to B antigen.
- If H gene absent (extremely rare), no H substance can be formed, and therefore no A or B antigen. Result is Bombay blood group.
Bombay blood group:
The Bombay blood group lacks H gene and therefore cannot make H antigen (H substance). Since the H substance is the precursor for the A and B antigens, these antigens also are not made. The cells type as O and the serum has anti-A, anti-B, and anti-H since the individual lacks all of these antigens. Anti-H agglutinates O cells. The only cells Bombay individuals do not agglutinate are from other Bombay blood people since they lack the H antigen,
Subgroups of A and B
The subgroups of A and B are caused by decreased amounts of antigen on the red blood cells. They are inherited conditions.
The most common are subgroups of A. Approximately 80% of the A's and AB's have a normal expression of A1. Most of the other 20% are either A2 or A2B. This subgroup has fewer H chains converted to A antigen – result is more H chains on red cell, and fewer A antigens. 
A small percentage of the individuals
A small percentage of the individuals
There are other, weaker subgroups of A exist: A3; Aint; Am, Ax; Ael. Each has a different pattern of reacting with anti-A, anti-A, and various antibody-like substances called lectins.
Lectins
Lectins are extracts of seeds of plants that react specifically with certain antigens. The two most common lectins used in Blood Bank are:
- Ulex europaeus, or lectin H, which agglutinates cells that have H substance.
- Dolichos biflouros, or lectin A1, which agglutinates cells with A1.
Lectin-H reacts strongest with O cells, which has a high concentration of H antigen, and weakest with A1 cells, which have a low concentration of H.
| Lectin | O cells | A2 cells | A2B cells | B cells | A1 cells | A1B cells | Bombay cells |
| lectin-H | 4+ | 3+ | 2-3+ | 2+ | weak to negative | weak to negative | negative |
| Lectin-A1 | negative | negative | negative | negative | positive | positive | negative |
Differentiating Subgroups of A:
Use the following steps to help differentiate the subgroups of A:
- Use lectin-A1 to differentiate A1 cells from all others - will agglutinate only A1 cells
- Look for weaker or mixed field reactions
- Look for anti-A1 in serum (serum reacts with A1 cells but not A2 cells)
- Look at strength of reactions with anti-A,B or with lectin-H
| GROUP | A1 | A2 | A3 | Ax |
| Reaction with anti-A | 4+ | 4+ | mf | 0 |
| Reaction with anti-A,B | 4+ | 4+ | mf | 2+ |
| Reaction with Lectin-A1 | 4+ | 0 | 0 | 0 |
| Reaction with Lectin-H | 0-w | 1-2+ | 2+ | 2-3+ |
| Presence of anti-A1 | no | may | may | often in serum |
Problems with Ax:
Because Ax cells initially type as O and serum usually has anti-A1, (along with anti-B), patient forwards and reverses as O. Unfortunately when Ax is transfused into an O individual, the naturally occurring anti-A,B will react with the donor cells causing a transfusion reaction. Therefore: to prevent Ax from being erroneously typed as O, confirm all group O donors with anti-A,B.
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