Page 301 - Color Atlas of Biochemistry
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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
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