Blood Groups and Haemostasis

Question 1

what determines blood type?

Answer 1

• presence/absence of surface antigens
• antigens signal to the immune system

Question 2

RBC antigens

Answer 2

• can react with its particular antibodies (ABs) if present in plasma
• AB - a protein produced by the immune system in response to a specific antigen
• each AB recognises a specific antigen unique to its target
• ABs can identify and neutralise foreign cells like bacteria and viruses
• over 50 types
• 3 important antigens: A,B, Rhesus
• proportions of these vary with different populations
• most common in western europe - O an A

Question 3

ABO System

Answer 3

• type A - surface antigen A - anti-B antibodies
• type B - surface antigen B - anti-A antibodies
• type AB - surface antigens A and B - neither anti-A nor anti-B antibodies
• type O - neither A nor B surface antigens - anti-A and anti-B antibodies

Question 4

Mixing of incompatible groups

Answer 4

• agglutination
• e.g. the anti- A antibodies in recipient combine withe the type A antigens on the red blood cells in the donated blood
• haemolysis - sometimes instead of antibodies causing agglutination they activate the complement system releasing proteolytic enzymes causing the red blood cells to rupture

Question 5

rhesus system

Answer 5

• rhesus positive - D-antigen present on red blood cell - no D-antibody
• rhesus negative - no D-antigen present on red blood cell - no D-antibody
• gives a total of 8 possible blood groups: O+, O-, A+, A- etc.

Question 6

Rhesus (Rh) Factor

Answer 6

• 3 main rhesus antigens on RBC: C, D (most common), E
• Rh- individuals normally have no anti-Rh antibodies - must be induced - part of immune reaction
• unlikely to be any Rh dependent agglutination following an initial transfusion with Rh+ blood
• problems occur when theres a repeated transfusion of Rh+ to Rh- recipient : immunity builds up, haemolysis and agglutination of donor RBCs

Question 7

universal donor and recipient?

Answer 7

• universal donor - O-
• universal recipient - AB+

Question 8

what is cross-matching?

Answer 8

a laboratory test to find if the body will accept blood from a donor - establishes the compatibility between donor and recipient and minimises risk of haemolytic reactions

Question 9

What is haemostasis and what are the three phases?

Answer 9

• prevention of blood loss

1.) vascular spasms - immediate vasoconstriction in response to injury
2.) platelet plug formation - platelet adhesion and aggregation
3.) coagulation - blood clotting

Question 10

phase 1 of haemostasis

Answer 10

vascular spasms

• vasoconstriction
• nervous reflexes
• myogenic contraction
• factors from damaged tissue and activated platelets e.g. endothelin, thromboxane A2, serotonin (5-HT)

Question 11

phase 2 of haemostasis

Answer 11

• platelet plug formation
• platelets are activated by contact with: von Willebrand factor (vWF), collagen, thrombin, a negatively charged surface
• platelets do not normally stick to the vessel wall
• endothelial surface: smoothness, glycocalyx, prostacyclin and NO, thrombomodulin (binds thrombin)
• activated platelets release: ADP, thromboxane A2, serotonin

Question 12

phase 3 of haemostasis

Answer 12

• coagulation
• a set of reactions in which blood is turned from liquid to gel
• follows intrinsic and extrinsic pathways as well as common pathway - extrinsic and intrinsic paths produced thromboplastin, which acts via the common pathway

Question 13

extrinsic pathway

Answer 13

• tissue factors released from damaged endothelial/tissue cells
• increased damage, increased tissue factor, faster clotting
• tissue factor + Ca + Factor VII produces tissue thromboplastin
• faster than intrinsic pathway

Question 14

intrinsic pathway

Answer 14

• exposed collagen at injury site activates enzymes
• aggregating platelets release a platelet factor
• these act with Ca2+ to drive a series of linked reactions to produce platelet thromboplastin

Question 15

common pathway

Answer 15

• thromboplastin (T or P) activates Factor X
• Factor X converts prothrombin to thrombin
• thrombin converts fibrinogen to fibrin

Question 16

Clot breakdown (fibrinolysis)

Answer 16

• damaged tissue releases Tissue Plasminogen Activator (t-PA)
• t-PA activates Plasminogen in plasma
• Plasminogen activates the enzyme Plasmin
• Plasmin dissolves fibrin strands - removes the foundation of the clot

t-PA in thrombolytic therapy
- heart attack
- stroke
- pulmonary embolism

Question 17

bleeding time

Answer 17

• duration of bleeding after controlled, standardised puncture of earlobe or forearm
• measure capillary and platelet function
• normal - 1-3 minnutes

Question 18

clotting time / coagulation time

Answer 18

• time required for blood to clot in a glass tube
• mainly reflects time required for generation of thrombin
• prolonged if plasma concentration of prothrombin or other clotting factors is low
• normal - 4-10 minnutes

Question 19

hypercoagulation - thrombi and emboli

Answer 19

• endothelial damage: arteriosclerosis, infection, trauma
• slowly flowing or static blood: leg immobilisation, atrial fibrillation
• myocardial infarction, stroke, deep vein thrombosis, pulmonary embolism

embolism - clot that travels from site where it was formed

thrombus - blood clot that forms in a vessel

Question 20

hypocoagulation

Answer 20

• deficiency of platelets - thrombocytopenia purpura
• deficiency of coagulation factors - haemophilia A (lack of factor VIII), haemophilia B (lack of factor IX)
• deficiency of vitamin K