Platelet Structure and Function

Question 1

What are platelets and what is their primary function?

Answer 1

Platelets are anucleate fragments of megakaryocyte cytoplasm. Their primary function is to mediate primary haemostasis (the formation of the initial platelet plug).

Question 2

Platelets are anucleate fragments of megakaryocyte cytoplasm. Their primary function is to mediate primary haemostasis (the formation of the initial platelet plug).

Answer 2

The average lifespan is 10 days (range 8-12). It is shortened by conditions that increase platelet consumption, such as thrombosis, infection, and splenic enlargement.

Question 2

Describe the three structural zones of a platelet and their basic components.

Answer 2

Describe the three structural zones of a platelet and their basic components.

Question 2

What is the normal platelet count range, and are there any population-based variations?

Answer 2

The normal range is 150–400 × 10⁹/L. A lower count is found in neonates (100–300 × 10⁹/L) and in certain racial populations (e.g., southern Europe or the Middle East).

Question 2

What is stored in the two main types of platelet granules?

Answer 2

Dense Granules: Contain small molecules like ADP, Ca²⁺, and serotonin.

Alpha (α) Granules: Contain proteins like platelet factor 4 (PF4), PDGF, β-thromboglobulin, fibrinogen, and other clotting factors.

Question 3

What are the key glycoprotein receptors on the platelet surface and their primary ligands?

Answer 3

GP Ia: Collagen

GP Ib: von Willebrand factor (vWF)

GP IIb/IIIa: Fibrinogen and vWF (this receptor links platelets together via fibrinogen).

Question 4

Name two important chemical mediators released/formed by activated platelets and their functions.

Answer 4

Thromboxane A2 (TxA2): Potentiates platelet aggregation and causes vasoconstriction.

ADP: Promotes platelet activation and aggregation.

Question 5

How is the severity of thrombocytopenia classified by platelet count?

Answer 5

Mild: 100 - 139 × 10⁹/L

Moderate: 50 - 99 × 10⁹/L

Severe: < 50 × 10⁹/L

Very Severe: < 20 × 10⁹/L

Question 6

What is pseudo-thrombocytopenia, and how is it resolved?

Answer 6

It is an artificially low platelet count caused by EDTA-mediated platelet clumping in the blood collection tube. It can be resolved by re-analyzing a blood sample collected in a citrated tube.

Question 6

What are the two main pathophysiological mechanisms that cause thrombocytopenia?

Answer 6

Decreased production by the bone marrow.

Increased consumption (or destruction) of platelets in the periphery.

Question 6

What key features in a patient’s history might suggest an inherited platelet disorder rather than an acquired one?

Answer 6

A life-long history of bleeding or a family history of similar platelet abnormalities suggests a heritable disorder.

Question 7

What is ITP?

Answer 7

ITP is an autoimmune disorder characterized by isolated thrombocytopenia (platelet count <100 × 10⁹/L) due to autoantibodies (usually against the GPIIb/IIIa receptor) that coat platelets, leading to their premature destruction by macrophages.

Question 8

What is the difference between Primary and Secondary ITP?

Answer 8

Primary ITP: Idiopathic, with no identifiable cause.

Secondary ITP: Associated with another condition, such as viral infections (Hepatitis C, HIV), autoimmune diseases (e.g., SLE), or lymphoproliferative disorders (e.g., CLL).

Question 8

How are “Persistent ITP” and “Chronic ITP” defined?

Answer 8

Persistent ITP: Lasting between 3 and 12 months after diagnosis.

Chronic ITP: Lasting for more than 12 months after diagnosis.

Question 8

How is a diagnosis of ITP made?

Answer 8

ITP is a diagnosis of exclusion. A bone marrow exam is not always needed but, when performed, typically shows normal or increased numbers of megakaryocytes without dysplasia.

Question 9

List the main treatment options for ITP.

Answer 9

First-line: Corticosteroids, Intravenous Immunoglobulins (IVIg), anti-D immunoglobulin.

Second-line/Refractory: Thrombopoietin receptor agonists (TPO-RAs), Rituximab (anti-CD20), Splenectomy, and other immunosuppressants (e.g., mycophenolate).

Question 10

What are the key clinical challenges in diagnosing Inherited Platelet Disorders (IPDs)?

Answer 10

IPDs are often misdiagnosed as ITP and may receive inappropriate treatment. They can present with thrombocytopenia, platelet dysfunction, or both, and may be part of a clinical syndrome requiring long-term monitoring.

Question 10

What is Glanzmann Thrombasthenia (GT)?

Answer 10

It is a rare, autosomal recessive disorder caused by a defect in the GPIIb/IIIa receptor. This means platelets cannot bind fibrinogen to clump together.

Lab Finding: Platelet count and size are normal, but aggregation is absent with all agonists except ristocetin.

Question 11

What is Bernard-Soulier Syndrome (BSS)?

Answer 11

It is an autosomal recessive disorder caused by a defect in the GPIb-IX-V complex (the receptor for vWF).

Key Triad: 1) Thrombocytopenia, 2) Giant platelets, and 3) Failure to bind vWF.

Lab Finding: Platelet aggregation is normal with all agonists except ristocetin (no response).

Question 12

What is the key diagnostic test to distinguish between Glanzmann Thrombasthenia and Bernard-Soulier Syndrome?

Answer 12

Flow cytometry can confirm the diagnosis by identifying the missing glycoprotein receptor:

Glanzmann’s: Deficiency of GPIIb (CD41) or GPIIIa (CD61).

Bernard-Soulier: Deficiency of GPIb (CD42).

Question 13

What is Wiskott-Aldrich Syndrome (WAS)?

Answer 13

It is an X-linked recessive disorder characterized by the triad of:

Micro-thrombocytopenia (small platelets)

Eczema

Immune dysregulation (leading to recurrent infections and autoimmune issues).

Question 14

Compare and contrast the vascular effects of Prostacyclin (from endothelial cells) and Thromboxane A2 (from platelets).

Answer 14

Thromboxane A2 (TxA2): Promotes platelet aggregation and is a potent vasoconstrictor.

Prostacyclin (PGI2): Inhibits platelet aggregation and is a potent vasodilator. The balance between these two is crucial for normal blood flow and hemostasis.

Question 14

A patient on aspirin needs surgery. What is the specific molecular mechanism by which aspirin increases bleeding risk?

Answer 14

Aspirin irreversibly inhibits cyclo-oxygenase (COX-1) in platelets. This prevents the formation of Thromboxane A2 (TxA2) , a potent platelet aggregator and vasoconstrictor. Since platelets are anucleate, they cannot synthesize new COX-1, so the effect lasts for their entire lifespan (7-10 days).

Question 14

Where are phospholipid-binding sites exposed, and why is this critical for the coagulation cascade?

Answer 14

Phospholipid-binding sites are exposed on the membrane of activated platelets. This is critical because these sites provide the surface for the assembly and activation of the tenase and prothrombinase complexes, which are essential for converting Factor X to Xa and prothrombin to thrombin.

Question 14

A 25-year-old woman with no past medical history presents with easy bruising and petechiae. Her CBC shows isolated thrombocytopenia (platelets 22 x 10⁹/L) with normal WBC and Hb. What is the most likely diagnosis, and what is the first step in management?

Answer 14

The most likely diagnosis is Immune Thrombocytopenia (ITP) . It is a diagnosis of exclusion in an otherwise well patient with isolated thrombocytopenia. First-line management for severe bleeding or very low counts often includes corticosteroids.

Question 15

A patient with a lifelong history of mucocutaneous bleeding (epistaxis, gum bleeding) has a normal platelet count and normal platelet size on blood film. What is the most likely category of disorder, and what is the next step in diagnosis?

Answer 15

A normal platelet count with bleeding history suggests a qualitative platelet function disorder, not a quantitative one (thrombocytopenia). The next step is platelet function testing, typically light transmission aggregometry (LTA) with various agonists.

Question 16

A child presents with thrombocytopenia, eczema, and recurrent infections. What is the likely diagnosis, and what is the key platelet finding on the blood film?

Answer 16

The likely diagnosis is Wiskott-Aldrich Syndrome (WAS) . It is an X-linked recessive disorder. The key finding on the blood film is small platelets (microthrombocytopenia) , which helps distinguish it from other inherited thrombocytopenias that often feature giant platelets.

Question 16

A patient with a normal platelet count has a prolonged bleeding time. Aggregometry shows no aggregation with ADP, collagen, or arachidonic acid, but a normal response to ristocetin. What is the diagnosis, and what is the molecular defect?

Answer 16

The diagnosis is Glanzmann Thrombasthenia (GT) . The molecular defect is in the GPIIb/IIIa (αIIbβ3 integrin) receptor, which is the final common pathway for platelet aggregation via fibrinogen binding. The preserved ristocetin response confirms that the GPIb-IX-V (vWF) receptor is working.

Question 17

A patient has moderate thrombocytopenia and giant platelets on the blood film. Aggregometry shows a normal response to all agonists except ristocetin. What is the diagnosis, and what is the deficient receptor?

Answer 17

The diagnosis is Bernard-Soulier Syndrome (BSS) . The deficient receptor is the GPIb-IX-V complex, which is the receptor for von Willebrand factor (vWF). The giant platelets are a characteristic morphological clue.

Question 18

What is the significance of the "fuzzy glycocalyx coat" on a platelet?

Answer 18

The glycocalyx is the outermost coat of the platelet and is responsible for adsorbing proteins from the plasma, including coagulation factors, which are then concentrated and activated on the platelet surface.

Question 19

What does the "canalicular system" do for a platelet?

Answer 19

The open canalicular system (OCS) is a network of channels connected to the surface membrane. It serves two main purposes: It provides a vast reactive surface area for absorbing coagulation factors. It is the route for release of granule contents during the platelet release reaction.

Question 20

When is a bone marrow examination indicated in the workup of thrombocytopenia?

Answer 20

A bone marrow exam is not routinely needed for typical ITP. It is reserved for: Patients with atypical presentations (e.g., presence of constitutional symptoms, abnormal WBCs or RBCs on film). Patients who are not responding to standard ITP treatment as expected, to rule out other causes like aplastic anemia or myelodysplasia.

Question 21

A lab report shows a low platelet count, but the patient has no bleeding symptoms and no history of bleeding. The blood film is reviewed and shows platelet clumping. What is the next step?

Answer 21

Suspect pseudo-thrombocytopenia due to EDTA. The next step is to check the platelet count using a citrated (blue-top) blood tube, which will resolve the clumping and give an accurate count.

Question 22

In the lab evaluation of a suspected Inherited Platelet Disorder, what is the role of flow cytometry?

Answer 22

Flow cytometry using antibodies against specific glycoproteins is used to confirm a diagnosis by identifying quantitative deficiencies: Glanzmann's: Absence/reduction of CD41 (GPIIb) or CD61 (GPIIIa) . Bernard-Soulier: Absence/reduction of CD42 (GPIb) . Note: It will not detect qualitative (dysfunctional) defects.

Question 23

What is the key diagnostic limitation of flow cytometry for Glanzmann Thrombasthenia?

Answer 23

Flow cytometry can detect the quantitative absence or reduction of the GPIIb/IIIa receptors (Type I or Type II GT). However, it will not detect qualitative defects, where the receptor is present but non-functional. Genetic testing may be needed in such cases.

Question 24

A patient's platelets aggregate normally with ristocetin but fail to secrete their granule contents in response to thrombin. What type of disorder is suspected, and which granule is most likely affected?

Answer 24

This suggests a storage pool disease (SPD) , a defect in the granules themselves. Based on the failure to secrete in response to a strong agonist like thrombin, it could be either δ-SPD (dense granule deficiency) or α-SPD (Gray Platelet Syndrome) . Further specific tests (electron microscopy, luminescence) would differentiate them.

Question 25

Besides anti-GPIIb/IIIa antibodies, against what other antigens can autoantibodies be directed in ITP?

Answer 25

While anti-GPIIb/IIIa are most common, antibodies in a minority of ITP patients may be directed against other platelet glycoprotein receptors, such as the GPIb-IX-V complex. Patients with anti-GPIb antibodies may respond less well to standard ITP treatments like IVIg.