Neutrophils: The neutrophils have a diameter of 12-15 µm. Their nucleus is divided into 2 - 5 lobes connected by a fine nuclear strand or filament. The cytoplasm is transparent due to the presence of small granules. Interestingly in the nucleus of the neutrophil from females, we can observe a Barr body which is the inactivated X chromosome (Figure 1).

Eosinophils: The eosinophils are quite rare in the blood. They have the same size as the neutrophils and generally have bilobed nucleus. The cytoplasm is granular which stains pink to orange with acidic dyes (Figure 1).

Basophils:  Basophils are the rarest leukocytes having a diameter of 9-10 µm. Their cytoplasm is granular which stains dark purple with basic dyes. Their nucleus is bi or tri-lobed.

Lymphocytes: Lymphocytes are 8-10 µm in diameter and generally they are smaller than the other leukocytes (Figure 1). The cytoplasm is transparent. The nucleus is round and large occupying most of the cellular space. With respect to the amount of cytoplasm, lymphocytes are divided into small, medium and large. The lymphocytes are the main constituents of the immune system which is a defense against the attack of pathogenic micro-organisms such as viruses, bacteria, fungi and protista. Lymphocytes yield antibodies and arrange them on their membrane. An antibody is a molecule able to bind itself to molecules of a complementary shape called antigens, and recognize them. Lymphocytes can be further divided into B and T cells and the natural killer cells. The natural killer cells are characterized by their cytotoxic activity. They kill viruses, bacteria, infected and neoplastic cells and also regulate the production of other hematic cells such as erythrocytes and granulocytes.

Monocytes:  Monocytes are the largest leukocytes having the diameter of 16-20 µm (Figure 1). They have a horseshoe-shaped nucleus with a transparent cytoplasm.  Most monocytes are the precursors of macrophages which are larger blood cells. In the presence of an inflammation site, monocytes quickly migrate from the blood vessel and start an intense phagocytory activity. They produce substances which have defensive functions such as lysozyme, interferons and other substances which modulate the functionality of other cells. Macrophages cooperate in the immune defense. They expose molecules of digested bodies on the membrane and present them to more specialized cells, such as B and T lymphocytes.