Leukocytes, or white blood cells, are the mobile defense force of the immune system. Produced in the bone marrow, they circulate throughout the blood and lymphatic tissues, neutralizing foreign invaders and damaged cells. Unlike the numerous red blood cells, leukocytes are less common but possess a nucleus. Microscopic examination of leukocytes is a routine part of a Complete Blood Count (CBC), providing insight into the body’s health status and immune response.
Preparing the Sample for Microscopic Analysis
Viewing leukocytes requires careful preparation, as they are transparent and colorless in their natural state. The process begins with creating a peripheral blood smear, spreading a drop of blood thinly across a glass slide. The smear is then fixed, typically with methanol, to preserve the cell structure.
The slide is stained using a polychromatic dye mixture, most commonly a Romanowsky stain like Wright-Giemsa. This stain uses acidic and basic dyes that bind to different cellular components. Acidic components, such as DNA in the nucleus, attract the basic dye (like methylene blue), staining the nucleus purple or blue. Basic components in the cytoplasm and granules attract the acidic dye (like eosin), resulting in pink or red staining. This differential staining allows for the visual identification of the five distinct types of leukocytes.
Visual Identification of White Blood Cell Types
The five types of leukocytes—Neutrophils, Lymphocytes, Monocytes, Eosinophils, and Basophils—are distinguished entirely by their morphology when stained and viewed under high magnification. These cells are generally much larger than the surrounding red blood cells, which serve as a helpful size reference. The primary distinguishing features are the size and shape of the nucleus and the presence or absence of clearly visible cytoplasmic granules.
Neutrophils
Neutrophils are the most abundant type, typically measuring around 12 to 15 micrometers in diameter. Their most recognizable feature is a multi-lobed nucleus, usually consisting of three to five segments connected by thin strands of nuclear material. The cytoplasm contains numerous, fine granules that stain a pale pink or light purple color, giving the cell a neutral appearance.
Lymphocytes
Lymphocytes are the second most common type, measuring 6 to 9 micrometers. The nucleus is large and spherical, often taking up nearly the entire cell volume and containing dense, dark-staining chromatin. The cytoplasm is typically very scant and appears as a thin, pale blue crescent surrounding the nucleus.
Monocytes
Monocytes are the largest leukocytes, ranging from 12 to 15 micrometers in diameter. These cells are characterized by a large, non-segmented nucleus that is often indented, appearing kidney-shaped or C-shaped. Monocytes have a relatively abundant amount of cytoplasm, which stains a characteristic dull gray-blue color. While they are classified as agranulocytes, their cytoplasm may contain fine, dust-like granules.
Eosinophils
Eosinophils are easily identified by their prominent, large granules that stain brightly red or reddish-orange due to the affinity for the acidic eosin component of the stain. The nucleus of an eosinophil is typically bi-lobed, meaning it has two segments connected by a thin strand, resembling a pair of spectacles. These cells are similar in size to neutrophils, falling within the 12 to 15 micrometer range.
Basophils
Basophils are the least common type, defined by large, coarse granules that stain deep blue or purple. These prominent granules are so numerous that they often completely obscure the underlying nucleus, making the nucleus shape difficult to discern. When visible, the nucleus is usually bi-lobed or S-shaped, and the cell size is similar to that of neutrophils and eosinophils.
Interpreting the Differential Count
The visual identification of these five cell types culminates in the differential count. This procedure involves counting a specific number of white blood cells, usually 100, and determining the percentage of each type present. The resulting percentages are compared against established reference ranges to detect abnormalities.
A total white blood cell count that is too high (leukocytosis) or too low (leukopenia) indicates an underlying health issue, such as infection, inflammation, or a bone marrow disorder. The diagnostic power lies in analyzing the shifts in the relative proportions of the individual cell types. For example, a disproportionate increase in neutrophils (neutrophilia) suggests an active bacterial infection or inflammation. Conversely, an elevated percentage of lymphocytes (lymphocytosis) is commonly associated with viral infections. A rise in the eosinophil count often points toward an allergic reaction or a parasitic infection.