Introduction
Blood typing is the classification of blood in terms of distinctive characteristics that are associated with the antigens located on the surface of red blood cells.
Blood Group Systems
About 30 antigens occur in the blood cells. Each of them can cause antigen-antibody reactions. Antigens present on RBCs are used to determine blood groups. More than 20 genetically determined blood group systems exist. Examples are; P, Lutheran, Lewis, XG, Duffy, Indian, Kidd, Diego, MNS, Kell, A-B-O, Rhesus, etc.
The most common blood group systems are the ABO and Rh blood grouping. This is because they are much more likely to cause blood transfusion reactions than the others.
Antigens (agglutinogens)
Antigens are oligosaccharides present on the red cell membrane. H-antigen is a precursor to the ABO blood group antigens. Antigens A & B determine the ABO blood group, while antigens C, D & E determine the Rh blood group present on red cell membranes.
Agglutinins (ANTIBODIES)
- Are γ globulins (immunoglobulin): IgM & IgG
- Originate from bone marrow and lymph glands
- Are not produced during foetal life
- Start appearing 2-3 months after birth
Abo Blood Grouping System
DISCOVERY: Austrian scientist – Karl Landsteiner in 1901; Found two antigens in RBCs and named them A & B antigens. Noticed corresponding antibodies in the serum and named them anti-A(α) antibody & anti-B(β) antibody.
Landsteiner’s law: if a particular antigen is present in the RBC, the corresponding Antibody must be absent in the serum, under normal conditions. The A, B & O blood groups are genetically determined based on the presence/absence of antigens A & B on the red cell membrane. Ags also present in other tissues like salivary gland, pancreas, kidney, lungs, liver and also in body fluids except CNS.
They appear in the sixth month of foetal life. Concentration at birth is one-fifth of that in adults. Four groups; A, B, AB & O. Inherited in a Mendelian fashion. Blood groups A & B are dominant while O is recessive.
| BLOOD TYPES WITH THEIR GENOTYPES AND THEIR CONSTITUENT AGGLUTINOGENS AND AGGLUTININS | GENOTYPES | BLOOD TYPES | AGGLUTNIOGENS | AGGLUTININS |
|---|---|---|---|---|
| OO | O | – | Anti-A & Anti-B | |
| OA or AA | A | A | Anti-B | |
| OB or BB | B | B | Anti-A | |
| AB | AB | A & B | – |
Rhesus Blood Grouping System
HISTORY: First discovered in rhesus monkey by LANDSTEINER & WEINER. Blood is commonly identified by an antigen called the Rhesus (Rh) factor. The "Rh factor" is named for the rhesus monkey. Antigens are primarily the C, D & E antigens. D is the most antigenic and is therefore used in determining the Rh blood type. Antigen D present – Rh-positive. Antigen D absent – Rh-negative. Has no corresponding natural Ab(antibody) in the serum. However, anti-D Ab is formed if a Rh- person is exposed to Rh+blood for the first time. Inherited in a dominant fashion:
- Homozygous – DD
- Heterozygous – Dd
- Complete absence – dd, leads to Rh-
RH ANTIBODIES
- IgG type
- Produced by exposure to foreign Rh antigen
RHESUS INCOMPATIBILITY
Occurs only when Rh-negative blood is exposed to Rh-positive blood through blood transfusion, birth, miscarriage, abortion, etc.
- First exposure – Primary response: Immunological memory
- Second exposure – immediate and severe response
- Rate of response increases with more exposure
| Contrast between abo and rh groups | ABO BLOOD GROUP | Rh BLOOD GROUP |
|---|---|---|
| It is present in tissues other than the red blood cells | It has not been detected in tissues other than the red blood cells | |
| Antigens are glycosylated | Antigens are not glycosylated | |
| Antigenic function is known | Antigenic function is not known | |
| Antibodies develop prior to exposure of a different antigenic individual | Anti-D antibodies do not develop after exposure of a D-negative individual | |
| Reaction to incompatibility is immediate | Reaction to incompatibility is relatively slow |
Importance Of Blood Typing
- Helps in the identification of antigens in red cell surface. Blood typing serves to promptly identify the cause of disorders such as Erythroblastosis foetalis, etc.
- Blood typing sometimes provides a method for resolving cases of disputed paternity
Clinical Correlates
- Blood transfusion
- Jaundice
- Cardiac shock
- Renal shut down
- Erythroblastosis foetalis (haemolytic Disease of Newborn)
- Hydrops foetalis
- Kernicterus
