Page 301 - Color Atlas of Biochemistry
P. 301
292 Tissues and organs
Fibrinolysis, blood groups possess. For example, carriers of blood group
A form antibodies against antigen B (“anti-B”),
while carriers of group B form antibodies
A. Fibrinolysis
against antigen A (“anti-A”). Individuals with
The fibrin thrombus resulting from blood blood group 0 form both types, and those
clotting (see p. 290) is dissolved again by with blood group AB do not form any of these
plasmin, a serine proteinase found in the antibodies.
blood plasma. For this purpose, the pre- If blood from blood group A is transfused
cursor plasminogen first has to be proteolyti- into the circulation of an individual with
cally activated by enzymes from various tis- blood group B, for example, then the anti-A
sues. This group includes the plasminogen present there binds to the A antigens. The
activator from the kidney (urokinase)and tis- donor erythrocytes marked in this way are
sue plasminogen activator (t-PA) from vascular recognized and destroyed by the complement
endothelia. By contrast, the plasma protein system (see p. 298). In the test tube, aggluti-
α 2 -antiplasmin, which binds to active plasmin nation of the erythrocytes can be observed
and thereby inactivates it, inhibits fibrinoly- when donor and recipient blood are incom-
sis. patible.
Urokinase, t-PA, and streptokinase, a bac- The recipient’s serum should not contain
terial proteinase with similar activity, are any antibodies against the donor erythro-
used clinically to dissolve thrombi following cytes, and the donor serum should not con-
heart attacks. All of these proteins are ex- tain any antibodies against the recipient’s
pressed recombinantly in bacteria (see erythrocytes. Donor blood from blood group
p. 262). 0 is unproblematic, as its erythrocytes do not
possess any antibodies and therefore do not
react with anti-A or anti-B in the recipient’s
B. Blood groups: the ABO system
blood. Conversely, blood from the AB group
During blood transfusions, immune reactions can only be administered to recipients with
can occur that destroy the erythrocytes trans- theAB group,as these arethe only ones with-
fused from the donor. These reactions result out antibodies.
from the formation of antibodies (see p. 300) In the Rh system (not shown), proteins on
directed to certain surface structures on the the surface of the erythrocytes act as antigens.
erythrocytes. Known as blood group antigens, These are known as“rhesus factors,”asthe
these are proteins or oligosaccharides that can system was first discovered in rhesus mon-
differ from individual to individual. More than keys.
20 different blood group systems are now The rhesusD antigen occursin 84% of all
known. The ABO system and the Rh system white individuals, who are therefore “Rh-pos-
are of particular clinical importance. itive.” If an Rh-positive child is born to an Rh-
In the ABO system, the carbohydrate parts negative mother, fetal erythrocytes can enter
of glycoproteins or glycolipids act as antigens. the mother’s circulation during birth and lead
In this relatively simple system, there are four to the formation of antibodies (IgG) against
blood groups (A,B,AB, and0). In individuals the D antigen. This initially has no acute ef-
with bloodgroupsAand B, theantigens con- fects on the mother or child. Complications
sist of tetrasaccharides that only differ in their only arise when there is a second pregnancy
terminal sugar (galactose or N-acetylgalactos- with an Rh-positive child, as maternal anti-D
amine). Carriers of the AB blood group have antibodies cross the placenta to the fetus even
both antigens (A and B). Blood group 0 arises before birth and can trigger destruction of the
from an oligosaccharide (the H antigen) that child’s Rh-positive erythrocytes (fetal erythro-
lacks the terminal residue of antigens A and B. blastosis).
The molecular causes for the differences be-
tween blood groups are mutations in the gly-
cosyl transferases that transfer the terminal
sugar to the core oligosaccharide.
Antibodies are only formed against anti-
gens that the individual concerned does not
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.