Leukocytes, commonly known as white blood cells, are diverse cells circulating within the blood and lymphatic system. They are a small fraction of blood cells, yet play a vital role in the body’s defense. They protect the body from various threats, and their characteristics provide valuable insights into a person’s health status.
The Role of Leukocytes in the Body
Leukocytes are integral components of the immune system, defending against invading pathogens and abnormal cells. They originate in the bone marrow and travel throughout the body. When an infection or injury occurs, these cells migrate to the affected site to neutralize threats. They identify and combat foreign invaders, such as bacteria, viruses, fungi, and parasites, and remove damaged or cancerous cells, maintaining overall health.
Preparing Blood for Microscopic Examination
To observe leukocytes under a microscope, a blood smear must first be prepared on a glass slide. This involves placing a small drop of blood on one end of a clean slide, then using another slide or a spreader to draw the blood across, creating a thin, even layer with a feathered edge. The blood smear must air dry completely before staining.
Staining techniques are employed to make the otherwise clear cells and their internal structures visible and distinguishable. Romanowsky stains, such as Giemsa or Wright’s stain, are commonly used for this purpose. These stains are neutral, composed of oxidized methylene blue (azure) dyes and eosin Y. The basic azure dyes bind to acidic components like the cell nucleus, producing blue-to-purple coloration, while the acidic eosin Y binds to alkaline cytoplasmic components, resulting in red, orange, or pink hues. This differential staining, known as the Romanowsky effect, allows for clear visualization of cellular morphology and differentiation between various cell types.
Identifying Different Types of Leukocytes
Observing a stained blood smear under a microscope allows for the identification of five main types of leukocytes. These types are broadly categorized into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Differences in nuclear shape, cytoplasmic characteristics, and granule appearance are key to their identification.
Neutrophils
Neutrophils are the most abundant type of white blood cell, making up 55% to 70% of all leukocytes. They are approximately 9 to 15 micrometers in diameter and have a multilobed nucleus, usually segmented into two to five lobes connected by thin filaments. Their cytoplasm stains a neutral pink or light lavender and contains fine, small granules that are generally not visible individually with a light microscope. Neutrophils are phagocytes, meaning they engulf and digest bacteria and other microorganisms.
Lymphocytes
Lymphocytes are the second most common leukocyte, accounting for 18% to 42% of white blood cells. They range from 6 to 15 micrometers in diameter, with mature cells typically smaller. Under a microscope, lymphocytes have a large, round, dark-staining nucleus that often occupies most of the cell, leaving a thin rim of pale blue cytoplasm. Lymphocytes are central to the adaptive immune system, with B cells producing antibodies and T cells directly attacking infected or abnormal cells.
Monocytes
Monocytes are the largest type of white blood cell, nearly twice the size of a red blood cell, with a diameter ranging from 15 to 22 micrometers. They possess a large, often kidney-shaped, bean-shaped, or horseshoe-shaped nucleus, which can appear lobulated or indented. The cytoplasm is abundant and appears grayish-blue, with some fine azurophil granules or vacuoles that may be occasionally visible. Monocytes differentiate into macrophages and dendritic cells in tissues, where they engulf foreign material and remove dead cells.
Eosinophils
Eosinophils make up 1% to 6% of leukocytes. Typically 12 to 17 micrometers in diameter, they are characterized by a bilobed nucleus, often resembling two separate raindrops connected by a thread. Their cytoplasm is packed with large, coarse, reddish or dark pink granules that stain intensely with eosin dye. Eosinophils mediate allergic reactions and defend against parasitic infections by releasing toxic proteins from their granules.
Basophils
Basophils are the least common leukocyte, making up less than 1% of white blood cells. Approximately 10 to 15 micrometers in diameter, they have a lobulated nucleus, often S-shaped or bilobed, which is frequently obscured by large, coarse, dark blue to purple granules in the cytoplasm. These granules contain histamine, heparin, and other chemical mediators that are released during inflammatory and allergic responses, contributing to symptoms like swelling and wheezing.
What Microscopic Examination Reveals
Microscopic examination of leukocytes provides valuable diagnostic information. Changes in the number, morphology, or maturity of these cells can indicate various health conditions. For instance, an abnormally high or low white blood cell count can signal an infection, inflammatory disorder, or blood cancers.
Specific changes in leukocyte morphology are also informative. The presence of coarse granules or vacuoles in neutrophils can suggest severe bacterial infections or active phagocytosis. Hypersegmented neutrophils, those with more than the usual number of nuclear lobes, may be observed in cases of vitamin B12 deficiency.
The appearance of immature leukocytes, known as blast cells, indicates conditions like leukemia. Furthermore, changes in the morphology of monocytes and lymphocytes have been associated with viral infections, such as COVID-19, where alterations in RNA content and cell size can be observed. This qualitative analysis complements quantitative blood counts, offering a complete picture for diagnosing and monitoring diseases.