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The classification of different types of white blood cells, or leukocytes, is primarily based on their staining characteristics. This refers to how these cells appear when treated with specific stains during microscopic examination. Different leukocytes take up stains in distinct ways due to variations in their cellular components, such as the presence of granules or the structure of the nucleus.

For instance, neutrophils, eosinophils, and basophils are classified as granulocytes because they have visible granules that can be differentiated when stained. Lymphocytes and monocytes, on the other hand, appear more homogeneous and generally do not have such granules, which allows for their classification as agranulocytes.

While factors such as cell size, the number of cells in a sample, and their functional abilities (like phagocytosis) play a role in the overall understanding of leukocyte function and abundance, the definitive method of categorization in histology relies on how these cells interact with stains, making staining characteristics the key characteristic for classification.