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Hematology

Function

Blood is often described as a “liquid organ” where each cellular component performs a specialized physiological task essential for survival. While the bone marrow handles production and the reticuloendothelial system handles destruction, the peripheral blood is the functional domain. Here, erythrocytes maintain tissue oxygenation, leukocytes provide immune surveillance and defense, and platelets ensure vascular integrity through hemostasis

For the laboratory scientist, understanding normal physiology is the baseline required to identify pathology. When these functions fail - whether through membrane defects, enzyme deficiencies, or receptor malfunctions - clinical disease states such as anemia, immunodeficiency, or coagulopathy arise

Erythrocyte Function: Gas Transport & Metabolism

The mature erythrocyte is an anucleate, biconcave disc designed for a singular primary purpose: the transport of respiratory gases. Despite lacking a nucleus and mitochondria, the RBC is metabolically active, relying on specific pathways to maintain its integrity and function

Hemoglobin Function & Oxygen Delivery

  • The Hemoglobin Molecule: The functional unit is a tetramer (primarily \(\alpha_2\beta_2\) in adults) containing four heme groups. Each heme contains ferrous iron (\(Fe^{2+}\)) capable of binding one molecule of oxygen
  • Cooperative Binding: Hemoglobin exhibits “allosteric” behavior. When the first oxygen molecule binds, the hemoglobin molecule changes shape (from “Tense” to “Relaxed”), making it easier for the subsequent three oxygen molecules to bind. This creates the characteristic sigmoidal oxygen dissociation curve
  • The Bohr Effect: Hemoglobin function is environmentally sensitive. In tissues with high metabolic activity (high \(CO_2\), low pH, high temperature), hemoglobin affinity for oxygen decreases (Right Shift), facilitating oxygen release to the tissues that need it most
  • Carbon Dioxide Transport: While some \(CO_2\) binds to hemoglobin (carbaminohemoglobin), the majority is converted to bicarbonate (\(HCO_3^-\)) by the RBC enzyme Carbonic Anhydrase. The bicarbonate diffuses out of the cell in exchange for chloride (the Chloride Shift), allowing massive amounts of \(CO_2\) to be carried in the plasma to the lungs

Essential Metabolic Pathways

Because the RBC cannot generate new proteins or use oxidative phosphorylation, it relies on four key pathways:

  • Embden-Meyerhof Pathway (Glycolysis): Generates ATP anaerobically. ATP is vital for powering the cation pumps (\(Na^+/K^+\) pump) that maintain cell volume and shape
  • Hexose Monophosphate Shunt: Produces NADPH via the enzyme G6PD. NADPH is essential for keeping glutathione reduced, which protects the cell from oxidative stress
  • Methemoglobin Reductase Pathway: Maintains iron in the ferrous (\(Fe^{2+}\)) state. If iron oxidizes to ferric (\(Fe^{3+}\)), it becomes Methemoglobin and cannot transport oxygen
  • Rapoport-Luebering Shunt: Produces 2,3-DPG. This molecule binds to hemoglobin and lowers its affinity for oxygen, encouraging oxygen release into tissues

Leukocyte Function: Innate & Adaptive Immunity

Leukocytes (White Blood Cells) are the mobile units of the body’s protective system. They function primarily outside the blood vessels, using the circulation merely as a highway to reach tissues via a process called diapedesis or extravasation

The Granulocytes (Innate Immunity)

  • Neutrophils: These are the “first responders” to bacterial infection. Their function relies on chemotaxis (moving toward chemical signals) and phagocytosis. Upon ingesting a bacteria, they utilize an oxidative burst (mediated by NADPH oxidase and Myeloperoxidase) to generate bleach-like compounds (hypochlorous acid) to kill the organism. They can also eject DNA webs called NETs (Neutrophil Extracellular Traps) to immobilize pathogens
  • Eosinophils: Specialized for killing large parasites (helminths) that are too big to be phagocytized. They degranulate, releasing toxic proteins like Major Basic Protein. They also dampen allergic responses by degrading histamine
  • Basophils: The initiators of allergic and inflammatory responses. Their granules contain histamine (vasodilator) and heparin (anticoagulant). They possess high-affinity IgE receptors; when antigens cross-link IgE on their surface, they degranulate explosively (anaphylaxis)

The Monocytes & Lymphocytes

  • Monocytes/Macrophages: Monocytes circulate briefly before entering tissues to become Macrophages. They are the “heavy lifters” of phagocytosis, clearing cellular debris and senescent cells. Crucially, they act as Antigen Presenting Cells (APCs), processing pathogens and displaying them to T-cells to initiate the adaptive immune response
  • T-Lymphocytes (Cell-Mediated)
    • CD4+ Helper T-cells: The “generals” of the immune system. They secrete cytokines that direct the activity of B-cells, macrophages, and CD8 cells
    • CD8+ Cytotoxic T-cells: The “assassins.” They scan cells for intracellular infections (viruses) or malignancy and induce apoptosis in the target cell via perforin and granzymes
  • B-Lymphocytes (Humoral): Upon activation, they differentiate into Plasma Cells, which are protein factories producing specific antibodies (immunoglobulins). These antibodies neutralize toxins, opsonize bacteria, and activate complement
  • Natural Killer (NK) Cells: A bridge between innate and adaptive immunity. They kill virally infected cells or tumor cells that try to hide by downregulating their surface markers (MHC I)

Platelet Function: Primary Hemostasis

Platelets are not merely cell fragments; they are complex metabolic packages responsible for the formation of the primary hemostatic plug. They patrol the vessel walls, monitoring for endothelial damage. Their function is a sequential cascade

The Sequence of Platelet Activation

  • Adhesion: When the endothelium is damaged, collagen is exposed. Von Willebrand Factor (vWF) binds to the collagen and acts as a molecular glue, grabbing the platelet via its GPIb/IX/V receptor
  • Activation: Binding triggers a shape change from a smooth disc to a spiky sphere (increasing surface area). The platelet membrane flips, exposing negative charges that facilitate the coagulation cascade
  • Secretion (Release Reaction): The platelet dumps its granules
    • Dense Granules: Release ADP (recruit more platelets) and Calcium
    • Alpha Granules: Release Fibrinogen, vWF, and Factor V
    • Thromboxane A2 (\(TXA_2\)): Synthesized from arachidonic acid (via Cyclooxygenase); causes vasoconstriction and further platelet aggregation
  • Aggregation: The final step where platelets stick to each other. This is mediated by the GPIIb/IIIa receptor, which binds fibrinogen. Fibrinogen links platelets together like mortar between bricks

Regulation

To prevent the blood from clotting inappropriately, the healthy endothelium secretes Nitric Oxide and Prostacyclin (\(PGI_2\)), which keep platelets in a resting, non-sticky state

Clinical Laboratory Assessment of Function

  • RBC Function: Assessed via indices (MCV, MCHC) and morphology. For example, spherocytes lack membrane surface area (Spectrin defect), while bite cells indicate enzyme failure (G6PD deficiency/oxidative damage)
  • WBC Function: While the CBC counts cells, morphology gives clues to function. Toxic granulation and vacuoles in neutrophils indicate active fighting of infection. Flow cytometry is used to assess lymphocyte subsets (CD4/CD8 ratios) and functional defects (e.g., adhesion disorders)
  • Platelet Function: The platelet count does not measure function. Functional testing includes:
    • PFA-100: Mimics high-shear blood flow to assess adhesion and aggregation time
    • Platelet Aggregometry: The gold standard. Agonists (ADP, Collagen, Ristocetin) are added to plasma to see if platelets clump. For example, in Glanzmann’s Thrombasthenia, platelets respond to Ristocetin but nothing else (defect in GPIIb/IIIa). In Bernard-Soulier, platelets fail to respond to Ristocetin (defect in GPIb)