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Rh Antigen

  1. The rhesus blood group antigens comprise of 5 antigens C, c, D, E and e. These antigens are located on short arm of chromosome 1.
  2. Most immunogenic among them is the Rh (0). Its presence or absence (0) designates a person as Rh positive or negative.
  3. Lewis and I antigen do not cause ery1hroblastosis fetalis and differ from all of the other red cell antigens in that they are not synthesized in the red cells membrane but are absorbed into it.
  4. Fetal Rh antigen are present by 38th day after conception.
  5. Although incompatibility for the major blood group antigens A and B is the most common cause of hemolytic disease in the newborn, the resulting anemia is usually very mild. About 20% of all infants have an ABO maternal blood group incompatibility, but only 5% are clinically affected.
  6. Most species of anti-A and anti-B antibodies are immunoglobulin M (IgM), which cannot cross the placenta and therefore cannot gain access to fetal ery1hrocytes. In addition, fetal red cells have fewer A and B antigenic sites than adult cells and are thus less immunogenic. The disease is invariably milder than O-isoimmunization and rarely results in significant anemia.

Definition:

Rh isoimmunization occurs when a Rh negative woman bears a Rh positive fetus. Normally, the fetal red cells containing the Rh antigen rarely enter the maternal circulation.

  • Pregnancy events causing fetal-maternal hemorrhage:

Event

Incidence (%)

Early pregnancy loss

3-5

Elective abortion

6-20

Ectopic pregnancy

5-8

Amniocentesis

4-11

Chorionic villus sampling

8-15

Cordocentesis

30-50

Antepartum trauma

Variable

Placental abruption

Low

Fetal demise

Variable

Manual placental extraction

Variable

External version

Variable


Although incompatibility for the major blood group antigens A and B is the most common cause of hemolytic disease in the newborn, the resulting anemia is usually very mild. About 20% of all infants have an ABO maternal blood group incompatibility, but only 5% are clinically affected.

Most species of anti-A and anti-B antibodies are immunoglobulin M (IgM), which cannot cross the placenta and therefore cannot gain access to fetal erythrocytes. In addition, fetal red cells have fewer A and B antigenic sites than adult cells and are thus less immunogenic. The disease is invariably milder than D-isoimmunization and rarely results in significant anemia.

CDE (Rhesus) Blood Group System

This system includes five red cell proteins or antigens: c, C, D, e, and E. No "d" antigen has been identified, and Rh- or D-negativity is defined as the absence of the D-antigen.

 

Red cell antigens and their propensity to cause hemolytic disease:

 

 

Blood Group System

Antigen

Severity of Hemolytic Disease

CDE (Rh)

D

C

c

E

E

Mild to severe with hydrops fetalis

Mild to moderate

Mild to severe

Mild to severe

Mild to moderate

I

 

Not a proven cause of hemolytic disease

Lewis

 

Not a proven cause of hemolytic disease

Kell

K

K

Mild to severe with hydrops fetalis

Mild to severe

Duffy

Fya

Fyb

Mild to severe with hydrops fetalis

Not a cause of hemolytic disease

Kidd

JKa

JKb

Mild to severe

Mild to severe

  1. Critical amount (fetal blood required to stimulate the maternal immune system to initiate the process of isoimmunization) of fetomaternal hemorrhage (FMH) = 0.1 mL.
  2. In case of Rh-negative mother, if the Rh-positive fetal cells enter the maternal system, then antibodies are formed against these antigens.
  3. Antibodies are of IgG and IgM variety, of which only IgG crosses the placenta and will cause fetal hemolysis.
  4. In Rh isoimmunization "outcome worsens with every pregnancy" (IgG antibodies increase in titers with every pregnancy).
  5. The first child is generally not affected because:
    1. The FMH occurs during delivery or late in pregnancy.
    2. Initially IgM-type antibodies are formed, which do not cross the placenta.
    3. The IgG antibodies are not present in sufficient titers.

Fetal Affection by the Rh antibody

The antibody formed in the maternal system (IgG) crosses the placental barrier and enters into the fetal circulation. The antibody will not have any effect on Rh-negative fetus. If the fetus is Rh-positive, the antibody becomes attached to the antigen sites on the surface of the fetal erythrocytes. The affected cells are rapidly removed from the circulation by the reticuloendothelial system. Depending upon the degree of agglutination and destruction of the fetal red cells, various types of fetal hemolytic diseases appear.

 


Extra Edge:
Clinical manifestations of the hemolytic disease of the fetus and neonate are:

  1. Hydrops fetalis
  2. Icterus gravis neonatorum
  3. Congenital anemia of the newborn




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