Leukocytes, or white blood cells, are colorless, nucleated cells that look noticeably larger than the red blood cells surrounding them on a blood smear. While red blood cells are small, uniform discs about 7 to 8 micrometers across, white blood cells range from 6 to 20 micrometers and come in five distinct types, each with a recognizable shape. To see their details, lab technicians apply a special stain that dyes acidic cell parts (like the nucleus) blue or purple and basic parts (like certain granules) red or orange. Without this stain, white blood cells would be nearly invisible.
Two Main Categories at a Glance
Under the microscope, white blood cells fall into two visual groups. Granulocytes have cytoplasm packed with visible, colored granules and multi-lobed nuclei that look segmented or pinched into sections. Agranulocytes have clear, smooth-looking cytoplasm and a single, unbroken nucleus. This difference is the first thing you notice when scanning a stained blood smear: some white blood cells look “busy” inside, and others look clean and simple.
The three granulocytes are neutrophils, eosinophils, and basophils. The two agranulocytes are lymphocytes and monocytes. Each has its own signature look.
Neutrophils: The Most Common Type
Neutrophils make up 55 to 70 percent of all white blood cells, so they’re the ones you’ll spot most often. They measure 12 to 14 micrometers across, roughly twice the diameter of a red blood cell. Their defining feature is a nucleus divided into 2 to 5 lobes connected by thin thread-like strands, giving the nucleus a segmented, almost bead-like appearance. The cytoplasm surrounding the nucleus is pale pink and filled with fine granules that are subtle compared to those in eosinophils or basophils.
Because their nucleus takes on so many shapes, neutrophils are sometimes called polymorphonuclear cells, which just means “many-shaped nucleus.” A younger neutrophil that hasn’t fully matured will have a thicker, band-shaped nucleus that hasn’t segmented yet. These are called band cells, and seeing a lot of them on a smear can signal that the body is pushing out white blood cells faster than normal in response to infection.
Eosinophils: Bright Red-Orange Granules
Eosinophils are easy to identify because their granules stain an intense red-orange color that stands out immediately. The individual granules are large enough to distinguish one from another, giving the cytoplasm a grainy, almost crystalline texture. The nucleus typically has just two lobes, often described as looking like a pair of sunglasses or a figure eight. These cells are 12 to 17 micrometers in diameter, about three times the size of a red blood cell, making them among the larger white blood cells. They account for only 1 to 4 percent of the total white blood cell count.
Basophils: Dark Granules That Hide the Nucleus
Basophils are the rarest white blood cell, making up just 0.5 to 1 percent of the total count, so finding one on a blood smear takes patience. At 14 to 16 micrometers, they’re a similar size to neutrophils. Their hallmark is coarse, dark blue-to-purple granules that are so large and densely packed they often completely obscure the nucleus underneath. Where a neutrophil’s granules are fine and easy to see past, a basophil’s granules dominate the entire cell. If you can’t clearly see the nucleus because dark granules are blocking it, you’re likely looking at a basophil.
Lymphocytes: Mostly Nucleus
Lymphocytes look strikingly different from granulocytes. Most are small, only 6 to 9 micrometers across, making them just slightly bigger than the red blood cells nearby. About 10 percent are larger, measuring 10 to 14 micrometers. What defines a lymphocyte is its nucleus-to-cytoplasm ratio: the dark purple, round nucleus takes up nearly the entire cell, leaving only a thin rim of pale blue cytoplasm around the edges. Some lymphocytes have so little visible cytoplasm that they appear to be almost all nucleus.
Small lymphocytes tend to look very uniform, so telling one from another is difficult. Larger lymphocytes show more variation and have a bit more visible cytoplasm. The nucleus itself can be round, oval, or slightly indented. Lymphocytes are the second most common type, representing 20 to 40 percent of white blood cells in a healthy adult.
Monocytes: The Largest White Blood Cell
Monocytes are the biggest white blood cells, reaching up to 20 micrometers in diameter. They have abundant grayish-blue cytoplasm with a characteristic “ground glass” appearance, meaning it looks slightly hazy or frosted rather than clear. Fine granules are scattered through the cytoplasm, and small bubble-like vacuoles are sometimes visible.
The nucleus is the key identifying feature. Instead of being round like a lymphocyte’s or segmented like a neutrophil’s, a monocyte’s nucleus is irregularly shaped, often described as kidney-bean or horseshoe-shaped. It folds and indents but doesn’t break into separate lobes. The chromatin inside the nucleus is slightly less condensed than in neutrophils, giving it a softer, more lace-like pattern. Monocytes make up 2 to 8 percent of white blood cells.
How Infection Changes Their Appearance
When the body is fighting a serious infection, white blood cells (particularly neutrophils) can develop visible changes that trained technicians look for on a smear. One common sign is toxic granulation: the granules inside neutrophils become larger and stain much darker blue or purple than normal. This happens because the cells are being produced and released faster than usual, shortening their maturation time. Toxic granulation shows up in roughly two-thirds to three-quarters of patients with severe bloodstream infections, though it can also appear with burns, trauma, or inflammation unrelated to infection.
Another visual clue is the appearance of Döhle bodies, which are pale blue, irregularly shaped inclusions that sit near the edges of the neutrophil’s cytoplasm. They represent clumps of internal cell machinery left over from rapid production. These show up in about 30 percent of patients with severe infections and are frequently seen alongside toxic granulation. They’re also common during pregnancy and the postpartum period, so their presence alone doesn’t confirm infection.
Quick Visual Comparison
- Neutrophil: 12 to 14 µm, multi-lobed nucleus (2 to 5 lobes), pale pink cytoplasm with fine granules
- Eosinophil: 12 to 17 µm, two-lobed nucleus, bright red-orange granules
- Basophil: 14 to 16 µm, nucleus hidden by coarse dark purple granules
- Lymphocyte: 6 to 14 µm, large round nucleus filling most of the cell, thin pale blue cytoplasm
- Monocyte: up to 20 µm, kidney-shaped or folded nucleus, grayish-blue “ground glass” cytoplasm