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PY2.1-13 | Haematology — Part 3

Platelets — Small Cells, Big Job (PY2.9)

Platelets (thrombocytes) are not complete cells — they are cytoplasmic fragments of giant bone marrow cells called megakaryocytes. Each megakaryocyte produces 2,000–3,000 platelets by shedding bits of its cytoplasm.

Figure: Platelets — Small Cells, Big Job (PY2.9)

Platelet characteristics:
• Count: 1.5–4.0 lakh/µL (150,000–400,000/µL)
• Size: 2–4 µm diameter (smallest 'cell' in blood)
• No nucleus, but contains: dense granules (ADP, serotonin, Ca²⁺), alpha granules (fibrinogen, vWF, platelet factor 4, PDGF), lysosomes, glycogen, mitochondria
• Lifespan: 8–12 days. Old platelets are destroyed in the spleen.
• Production regulated by thrombopoietin (TPO) — produced mainly by the liver

Platelet functions in haemostasis:
1. Adhesion — platelets stick to exposed collagen in a damaged vessel wall, mediated by von Willebrand factor (vWF) acting as a bridge between collagen and platelet receptor GPIb
2. Activation — adhered platelets change shape (from disc to spiny sphere), release granule contents (ADP, thromboxane A₂), and expose phospholipid surface (platelet factor 3) for coagulation
3. Aggregation — activated platelets recruit more platelets via ADP and thromboxane A₂. Platelets link together via fibrinogen bridges between GPIIb/IIIa receptors — forming the platelet plug

Clinical connections:
• Thrombocytopenia (< 1.5 lakh/µL): risk of spontaneous bleeding, petechiae, purpura. Causes: ITP (autoimmune), dengue, leukaemia, drug-induced
• Aspirin irreversibly inhibits cyclooxygenase (COX-1) in platelets → blocks thromboxane A₂ production → reduced aggregation for the life of the platelet (8–12 days). This is why low-dose aspirin prevents heart attacks.
• Clopidogrel blocks the ADP receptor (P2Y12) on platelets → reduces activation and aggregation

Haemostasis and the Coagulation Cascade (PY2.10, PY2.11)

Haemostasis = the process that stops bleeding. It occurs in three overlapping stages:

Stage 1: Vascular spasm
Injury to a blood vessel triggers immediate vasoconstriction (narrowing). Mechanisms: direct smooth muscle contraction, reflex neurogenic constriction, and local release of endothelin from damaged endothelium. This reduces blood flow to the area.

Stage 2: Platelet plug formation (primary haemostasis)
As described above: adhesion → activation → aggregation. The platelet plug is sufficient to seal small vessel injuries. This is tested by bleeding time (normal: 2–7 minutes).

Stage 3: Coagulation cascade (secondary haemostasis)
A series of enzyme reactions that converts soluble fibrinogen into an insoluble fibrin mesh, reinforcing the platelet plug into a stable clot.

The cascade has three pathways, all converging on the common pathway:

Intrinsic pathway (all factors are within the blood):
Exposed collagen → activates Factor XII (Hageman factor) → XII activates XI → XI activates IX → IX + VIIIa (cofactor) + Ca²⁺ + platelet phospholipid (PF3) = tenase complex → activates Factor X
Tested by: aPTT (activated partial thromboplastin time). Normal: 25–35 seconds

Extrinsic pathway (needs tissue factor from outside the blood):
Damaged tissue releases Tissue Factor (TF = Factor III) → TF + Factor VII + Ca²⁺ → activates Factor X
Tested by: PT (prothrombin time) / INR. Normal PT: 11–16 seconds
This pathway is the primary initiator of coagulation in vivo

Common pathway:
Factor Xa + Factor Va (cofactor) + Ca²⁺ + platelet phospholipid = prothrombinase complex → converts Prothrombin (Factor II) → Thrombin (Factor IIa) → Thrombin converts Fibrinogen (Factor I) → Fibrin monomers → Factor XIIIa (activated by thrombin) cross-links fibrin → Stable fibrin clot

Thrombin is the central enzyme — it has multiple roles:
• Converts fibrinogen → fibrin
• Activates factors V, VIII, XI, XIII (positive feedback — amplifies the cascade)
• Activates platelets
• Activates protein C (negative feedback — limits clotting)

Vitamin K is essential for synthesis of factors II, VII, IX, X (and proteins C and S) in the liver. Warfarin inhibits vitamin K recycling → used as an anticoagulant.

Mnemonic for vitamin K-dependent factors: "1972" = factors 10, 9, 7, 2.

Fibrinolysis and Natural Anticoagulants (PY2.12)

The body must balance clotting with clot dissolution — otherwise, every clot would grow uncontrollably. Two systems prevent this:

1. Natural anticoagulants (prevent excessive clotting):

• Antithrombin III (AT-III) — the most important natural anticoagulant. It inactivates thrombin, Xa, IXa, and other serine protease clotting factors. Heparin (both endogenous and therapeutic) works by massively accelerating AT-III's action (1000-fold increase).

• Protein C and Protein S — activated protein C (with protein S as cofactor) inactivates factors Va and VIIIa — the two key cofactors that amplify the cascade. Protein C is activated by thrombomodulin-thrombin complex on endothelial cells. This is a beautiful negative feedback: thrombin, the clotting enzyme, activates protein C, which then shuts clotting down.

• Tissue factor pathway inhibitor (TFPI) — inhibits the TF-VIIa-Xa complex, limiting the extrinsic pathway

• Intact endothelium — the endothelial surface produces:
• Prostacyclin (PGI₂) — inhibits platelet aggregation and causes vasodilation
• Nitric oxide (NO) — inhibits platelet adhesion and aggregation
• Heparan sulphate — surface GAG that activates AT-III
• Thrombomodulin — binds thrombin and redirects it to activate protein C

2. Fibrinolysis (dissolves the clot after healing):

The fibrinolytic system breaks down fibrin once it has served its purpose:

• Plasminogen (inactive) → activated to plasmin (active enzyme) by:
• Tissue plasminogen activator (tPA) — released from endothelial cells
• Urokinase plasminogen activator (uPA) — from kidneys, urine

• Plasmin digests fibrin into fibrin degradation products (FDPs), including D-dimers — which are clinically measured

• D-dimer — a specific FDP from cross-linked fibrin. ↑ D-dimer suggests recent clot formation and lysis. Used clinically to screen for DVT (deep vein thrombosis) and pulmonary embolism.

Anti-fibrinolytic agents (e.g., tranexamic acid) block plasmin and are used to treat excessive bleeding (trauma, heavy menstrual bleeding). Thrombolytic agents (tPA, streptokinase) activate plasminogen and are used to dissolve clots in acute MI and stroke.

Blood Groups and Transfusion (PY2.13)

Blood groups are determined by antigens on the RBC surface and antibodies in the plasma. Getting this wrong during transfusion can kill a patient within minutes.

The ABO System:
Gene on chromosome 9 encodes glycosyltransferases that add sugar residues to a precursor (H antigen) on the RBC surface:

Blood Group	Antigen on RBC	Antibody in Plasma	Can Donate To	Can Receive From
A	A antigen	Anti-B	A, AB	A, O
B	B antigen	Anti-A	B, AB	B, O
AB	A and B	Neither	AB only	A, B, AB, O (universal recipient)
O	Neither (H antigen only)	Anti-A and Anti-B	A, B, AB, O (universal donor)	O only

The antibodies are naturally occurring IgM — they appear by 6 months of age (from exposure to environmental antigens similar to A and B).

The Rh System:
The D antigen is the most important Rh antigen. You are either Rh-positive (D antigen present, ~85%) or Rh-negative (D antigen absent, ~15%).

Unlike ABO, Rh antibodies are NOT naturally occurring — they develop only after exposure (transfusion or pregnancy).

Rh incompatibility in pregnancy — Haemolytic Disease of the Newborn (HDN):
1. Rh-negative mother carries an Rh-positive fetus (father is Rh-positive)
2. During first pregnancy/delivery, fetal RBCs enter maternal circulation → mother produces anti-D IgG antibodies (sensitisation)
3. In a subsequent Rh-positive pregnancy, maternal IgG anti-D crosses the placenta → attacks fetal RBCs → haemolysis → anaemia, jaundice, hydrops fetalis
4. Prevention: Anti-D immunoglobulin (RhoGAM) given to Rh-negative mothers at 28 weeks and within 72 hours of delivery — destroys any fetal RBCs before the mother can mount an immune response.

Blood typing and cross-matching:
• Forward typing: Patient's RBCs + known anti-A and anti-B sera → which antigens are present?
• Reverse typing: Patient's serum + known A and B cells → which antibodies are present?
• Cross-matching: Donor RBCs + recipient serum (major cross-match) → any agglutination = incompatible

Transfusion reactions:
• Acute haemolytic reaction (most dangerous) — ABO-incompatible blood → IgM activates complement → intravascular haemolysis → haemoglobinaemia, haemoglobinuria, DIC, renal failure. Usually due to clerical error (wrong patient, wrong blood). Prevention: check, check, and check again.
• Febrile non-haemolytic — anti-leucocyte antibodies. Treat with paracetamol.
• Allergic — urticaria, due to plasma protein antibodies. Treat with antihistamine.
• Delayed haemolytic — anamnestic response to minor antigens. Occurs 2–10 days post-transfusion.
• TRALI (transfusion-related acute lung injury) — donor antibodies activate recipient neutrophils in pulmonary capillaries.