Hematology: The Study of Blood and Blood-Forming Tissues

Introduction to Hematology

Hematology is the branch of biology and medicine concerned with the study of blood, blood-forming tissues, and the formed elements of blood. This field is essential for understanding the composition and function of blood in health and disease.

• Formed elements of blood: These include red blood cells (RBCs), white blood cells (WBCs), and platelets (PLTs).

• Quantitative analysis: Refers to counting the number of each type of cell present in the blood.

• Qualitative analysis: Involves examining the appearance and morphology of each cell type.

Leukocytes (White Blood Cells)

General Characteristics of Leukocytes

Leukocytes are a diverse group of cells that play a crucial role in the immune system. They are generally larger and more complex than red blood cells and are characterized by the presence of a nucleus surrounded by cytoplasm.

• Staining: Wright stain is commonly used to differentiate blood cells:

  • RBCs: Stain red

  • WBCs: Have a purple nucleus with bluish cytoplasm

  • PLTs: Appear as small purplish dots

• Peripheral blood: Normally, only mature cells are seen in peripheral blood.

Types of Leukocytes

Leukocytes are classified based on their appearance and function. The main types include neutrophils, eosinophils, basophils, monocytes, and lymphocytes.

Neutrophils

• Function: Phagocytosis and destruction of bacteria; granules contain digestive enzymes.

• Appearance: Segmented nucleus (mature), band-shaped nucleus (immature), cytoplasm with fine granules.

• Normal range: 35-80% of circulating WBCs.

• Clinical significance: Increased neutrophils (neutrophilia) often indicate bacterial infection; decreased neutrophils (neutropenia) may result from bone marrow suppression or certain infections.

Eosinophils

• Function: Active in allergic reactions and defense against parasitic infections.

• Appearance: Slightly larger than neutrophils; cytoplasm contains large, red-orange granules; nucleus is bilobed.

• Normal range: 0-5% of circulating WBCs.

• Clinical significance: Increased eosinophils (eosinophilia) are associated with allergies, parasitic infections, and some drug reactions.

Basophils

• Function: Release histamine and heparin during inflammatory and allergic responses.

• Appearance: Large, blue-black granules obscure the nucleus; nucleus is often irregularly shaped.

• Normal range: Very low numbers in peripheral blood (0-1%).

• Clinical significance: Increased basophils may be seen in certain allergic or hypersensitivity disorders.

• Note: Tissue basophils are called mast cells and are similar but not identical to circulating basophils.

Monocytes

• Function: Phagocytosis; differentiate into macrophages in tissues.

• Appearance: Largest normal leukocyte (12-18 μm); nucleus is large, often indented or horseshoe-shaped; cytoplasm is abundant, blue-gray, and may contain vacuoles.

• Normal range: 2-10% of circulating WBCs.

Lymphocytes

• Function: Central to adaptive immunity; includes B cells, T cells, and natural killer (NK) cells.

• Appearance: Small (6-9 μm) or large (10-14 μm); round nucleus with scant blue cytoplasm.

• Normal range: About 34% of leukocytes in adults.

• Clinical significance: Increased lymphocytes (lymphocytosis) are often seen in viral infections.

Thrombocytes (Platelets)

Platelet Structure and Function

Platelets are small, anuclear cell fragments derived from megakaryocytes in the bone marrow. They play a vital role in hemostasis (blood clotting) and maintaining the integrity of blood vessels.

• Size: 1.5 to 4 μm in diameter.

• Appearance: Colorless to pale-blue background with centrally located, purplish-red granules.

• Function: Form platelet plugs to stop bleeding and release factors that promote clot formation.

Quantitative Evaluation of Blood Cells

Complete Blood Count (CBC)

The CBC is a routine hematology test that quantifies RBCs, WBCs, and platelets. It provides essential information for diagnosing anemia, infections, and other hematologic conditions.

• RBC count: Used to diagnose and monitor anemia.

• WBC count: Assesses immune status and detects infections or leukemias.

• PLT count: Evaluates clotting ability.

Manual Cell Counting: The Hemacytometer

A hemacytometer is a precision glass or plastic slide used for manual cell counts under a microscope. It contains two ruled areas with precise dimensions for accurate measurement.

• Counting areas: Each ruled area is 3 mm x 3 mm; depth is 0.1 mm.

• Counting procedure: Cells are counted in specific squares depending on the dilution used.

• Common dilutions:

  • RBC: 1:200 with isotonic fluid

  • WBC: 1:100 or 1:20 with ammonium oxalate

  • PLT: 1:100 with ammonium oxalate

Steps in Manual Cell Counting

1. Dilute blood sample with appropriate diluting fluid.

2. Load hemacytometer by capillary action.

3. Allow cells to settle for 2 minutes.

4. Place hemacytometer on microscope and focus on ruled area.

5. Count cells in a systematic pattern (snake-like) to avoid double counting.

6. Apply boundary rules: count cells touching left/top lines, not right/bottom lines.

7. Count both sides, sum totals, and average.

Cell Counting Calculations

• General formula for cell count:

• Example for RBC count:

  • Average count = 510

  • Dilution factor = 200

  • Area counted = 0.2 mm2

  • Depth = 0.1 mm

  • cells/μL

• Example for WBC count:

  • Average count = 58

  • WBC = [avg + (0.1 × avg)] × 100

  • cells/μL

Normal Ranges for Blood Cells

Normal values for blood cell counts vary by age, sex, and laboratory standards. The following table summarizes typical adult reference ranges:

Additional info: Normal ranges may vary slightly between laboratories and populations.