Ans. D. Codominant. (Ref. Harrison's principles of internal medicine 17th/pg. 708).Harrison's principles of internal medicine l7th/pg. 708........."The genes that determine the A and B phenotypes are found on chromosome 9p and are expressed in a Mendelian codominant manner""Pseudodominance is the sudden appearance of a recessive phenotype in a pedigree, due to deletion of a masking dominant gene. In this phenomenon One parent is unaffected heterozygote and another is affected homozygote. E.g. of diseases with pseudodominance include - Stargardt disease, Pseudoxanthoma Elasticum and Tarui's disease.ABO ANTIGENS AND ANTIBODIES# Th (1991), the most important in transfusion medicine.# The major blood groups of this system are A, B, AB, and O.# H substance is the immediate precursor on which the A and B antigens are added.# This H substance is formed by the addition of fucose to the glycolipid or glycoprotein backbone.# The subsequent addition of N-acetylgalactosamine creates the A antigen, while the addition of galactose produces the B antigen.# The genes that determine the A and B phenotypes are found on chromosome 9p and are expressed in a Mendelian codominant manner.# Individuals who lack the "A" and "B" transferases are phenotypically type "O," while those who inherit both trans- ferases are type "AB."# Rare individuals lack the H gene, which codes for fucose transferase, and cannot form H substance. These individuals are homozygous for the silent h allele (hh) and have Bombay phenotype (Oh).# The ABO blood group system is important because essentially all individuals produce antibodies to the ABH carbo- hydrate antigen that they lack.# Thus, persons with type AB are "universal recipients" because they do not have antibodies against any ABO phenotype, while persons with type O blood can donate to essentially all recipients because their cells are not recognized by any ABO isoagglutinins.# Nonsecretors are susceptible to a variety of infections (e.g., Candida albicans, Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae) as many organisms may bind to polysaccharides on cells. Soluble blood group antigens may block this binding.Rh SYSTEM# The Rh system is the second most important blood group system in pre- transfusion testing.# The Rh antigens are found on a 30- to 32-kDa RBC membrane protein that has no defined function.# The presence of the D antigen confers Rh "positivity," while persons who lack the D antigen are Rh negative.# Two allelic antigen pairs, E/e and C/c, are also found on the Rh protein.# The three Rh genes, E/e, D, and C/c, are arranged in tandem on chromosome 1 and inherited as a haplotype, i.e., cDE or Cde. Two haplotypes can result in the phenotypic expression of two to five Rh antigens.# The D antigen is a potent alloantigen.# About 15% of individuals lack this antigen.OTHER BLOOD GROUP SYSTEMS AND ALLOANTIBODIES# Antibodies to Lewis system carbohydrate antigens are the most common cause of incompatibility during pretransfusion screening. The Lewis gene product is a fucosyl transferase and maps to chromosome 19. Antibodies to Lewis antigens are usually IgM and cannot cross the placenta.# I system antigens are also oligosaccharides related to H, A, B, and Le. I and i are not allelic pairs but are carbohydrate antigens that differ only in the extent of branching. Some patients with cold agglutinin disease or lymphomas can produce anti-I autoantibodies that cause RBC destruction. Occasional patients with mononucleosis or Mycoplasma pneumonia may develop cold agglutinins of either anti-I or anti-i specificity. Administration of warm blood prevents isoagglutination.# The P system is another group of carbohydrate antigens controlled by specific glycosyltransferases. Its clinical significance is in rare cases of syphilis and viral infection that lead to paroxysmal cold hemoglobinuria. In these cases, an unusual autoantibody to P is produced that binds to RBCs in the cold and fixes complement upon warming. Anti- bodies with these biphasic properties are called Donath-Landsteiner antibodies. The P antigen is the cellular receptor of parvovirus B19 and also is a receptor for E.coli binding to urothelial cells.# The MNSsU system is regulated by genes on chromosome 4. M and N are determinants on glycophorin A, an RBC membrane protein, and S and s are determinants on glycophorin B. Anti-S and anti-s IgG anti- bodies may develop after pregnancy or transfusion and lead to hemolysis. Anti-U antibodies are rare but problematic; virtually every donor is incompatible because nearly all persons express U.# The Kell protein is very large (720 amino acids), and its secondary structure contains many different antigenic epitopes. The immunogenicity of Kell is third behind the ABO and Rh systems. The Kx gene is linked to the 91-kDa component of the NADPH-oxidase on the X chromosome, deletion or mutation of which accounts for about 60% of cases of chronic granulomatous disease.# The Duffy antigens are codominant alleles, Fya and Fyb, that also serve as receptors for Plasmodium vivax. More than 70% of persons in malaria-endemic areas lack these antigens.# The Kidd antigens. Jka and Jkb, may elicit antibodies transiently. A delayed hemolytic transfusion reaction that occurs with blood tested as compatible is often related to delayed appearance of anti-Jka.Additional Educational points:# The ABO blood system is the classic example of multiple allelism.# The ABO blood group substances are the best-known examples of human polymorphisms.# Carbohydrates present in blood group antigen is Fucose.0# ABO antigens are not found in CSF.0# The Rh ags are widely distributed in the tissues but are not found in body fluid, except the amniotic fluid.# Compared to FFP, for cryoprecipitate ABO incompatibility may not required to be checked for. 0# ABO incompatibility is the most common cause of hemolytic disease of the newborn.# For unexplained reasons, susceptibility to cholera is significantly influenced by ABO blood group status; those with type O blood are at greatest risk, while those with type AB are at least risk.# HLA is on Short arm of Chromosome 6.0# Febrile Non-Hemolytic Transfusion Reaction (FNHTR) is due to abs directed against donor leukocyte & HLA Ag.- If an Rh negative person has never before been exposed to Rh positive blood, transfusion of Rh-positive blood into that person will likely cause no immediate reaction. However, anti-Rh antibodies can develop in sufficient quantities during the next 2 to 4 weeks to cause agglutination of those transfused cells that are still circulating in the blood. These cells are then hemolyzed by the tissue macrophage system. Thus, a delayed transfusion reaction occurs, although it is usually mild. On subsequent transfusion of Rh-positive blood into the same person, who is now already immunized against the Rh factor, the transfusion reaction is greatly enhanced and can be immediate and as severe as a transfusion reaction caused by mismatched type A or B blood.