Leukocytes are classified into two groups based on the presence or absence of cytoplasmic granules. Granular leukocytes, which contain granules, belong to the myeloid lineage and are divided into three subtypes: neutrophils, eosinophils, and basophils. These cells are roughly spherical and characterized by the granules in their cytoplasm.

Neutrophils are the most abundant type of granular leukocytes, comprising 50-70% of all leukocytes. They feature small, evenly distributed granules and a polymorphonuclear appearance with two to five nuclear lobes. Neutrophils primarily function in the phagocytosis of bacteria.

Eosinophils and basophils both have a two-lobed nucleus. Eosinophils possess large granules that contain digestive enzymes, which are crucial in combating multicellular parasites such as flatworms. Conversely, basophils are marked by a dense cloud of histamine-containing granules surrounding their nucleus, playing a significant role in allergic reactions.

Monocytes, agranular leukocytes from the myeloid lineage, have a kidney-shaped nucleus and a cytoplasm rich in lysosomes. They migrate from the blood into tissues, where they differentiate into macrophages.

Lymphocytes, another type of agranular leukocytes from the lymphoid lineage, include B-cells, T-cells, and natural killer cells. Each plays a vital role in the immune system: B-cells produce antibodies, T-cells directly attack infected cells, and natural killer cells can perform both functions.