You are now at the beginning of the WBC Basics study section.

Leukocytes or WBCs are found in a thin gray layer known as the buffy coat in centrifuged blood. Above the leukocytes lie the platlets.

Leukocytes are primarily defensive, but also have important sanitation and recycling duties.

Before reviewing the origin and development of WBCs let us first look at the mature WBC population normally found in the peripheral blood (buffy coat), including neutrophils, eosinophils, basophils, monocytes, and lymphocytes.

Neutrophils The mature neutrophil (12-15u dia) is characterized by segmentation into 2-5 lobes. The chromatin is dense and clumped with distinct lighter areas of parachromatin.

The cytoplasm is lightly eosinophilic with variable numbers of light staining "neutral" granules and a few and a few azurophilic 1 granules, persistent from earlier stages.

Neutrophils are important in the inflammatory process (as phagocytes and mediators of inflammatory reactions).

The term neutrophil refers to the cell type not to the degree of differentiation.

Eosinophils The large orange granules of the eosinophil make the eosinophil the most readily recognizable cell in the blood. The eosinophil (12-15u diameter has chromatin similar to that of a neutrophil, but usually fewer (2-3) lobes.

Normally 0-6% eosinophils are found in the peripheral blood.

The eosinophil specific granules contain rhomboid crystals by EM. This core contains Major Basic Protein (MBP), known to be toxic to several parasites (helminths, microfilariae; schistomiasis), and some mammalian cells.

Compare the granules of the neutrophil (at left) with those of the eosinophil (at right).

Basophil The purple-black, often large coarse irregularly sized granules of the basophil are chacteristic and may obscure the nucleus of this relatively uncommon cell. The mature nucleus is segmented into 2-3 lobes.

Basophils (12-15u dia) contain heparin and large amounts of histamine.

Basophils are active participants in hypersensitivity reactions.

The mature nucleus is segmented into 2-3 lobes.

Monocytes are the largest (12-20u dia) cells normally found in the peripheral blood. Monocytes have abundant blue-gray, sometimes very pale pink cytoplasm with small, generally indistinct, granules. One may see fine reddish granules of variable prominence. Occasionally, large azurophilic granules will be seen.

Cytoplasmic vacuoles are often seen. The nucleus is irregular, frequently with delicate folds and often lobulated. The chromatin is fine and lacey, sometimes described as reticular.

The mature monocyte has no nucleoli.

Monocytes are phagocytes and frequently contain vacuoles. Monocytes can migrate into tissues where they are known as macrophages.

Monocytes are central to the "battle" management of inflammatory processes.

The peripheral blood normally contains 0-l0% monocytes.

Compare the chromatin and the size of the monocyte (top) with the lymphocyte.

Lymphocytes vary greatly in size (7-20m dia) and in nuclear and cytoplasmic character. The small lymphocyte (7-12m dia) has a round-oval nucleus with dense clumped "smudgy" chromatin; no visible nucleolus. Scanty light blue cytoplasm, while usually agranular, may contain a few small red granules. Contrast the chromatin of the lymphocyte and neutrophil. The lymphocyte at right is 15-18m dia.

Normal peripheral blood lymphocyte percentages vary from l5-60%.

Lymphocytes with peripherially clumped chromatin and often deep blue cytplasm similar to plasma cells are termed plasmacytoid lymphocytes.

Terminally differentiated lymphocytes or plasma cells (left panel) are rarely found in the peripheral blood.

Young or stimulated lymphocytes (right panel) are larger and have relatively more cytoplasm and larger nuclei than mature or unstimulated cells.

Some larger lymphocytes with moderate amounts of cytoplasm and distinct red granules are known as large granular lymphocytes (LGL).

Knowledge of the relative and absolute numbers of the various WBCs in the peripheral blood (PB) together with a qualitative evaluation of the morphology is key to assessing leukocytes.

The morphologic identification of a consecutive series of WBCs, by eye or automated instrument, results in a differential count, in which the relative number of each cell is expressed as a percentage. This may be deceiving.

Therefore, more important than the "Diff" is the calculation of the absolute count.

While examining your own blood you find:

a WBC (8,000 µ L) with	25 % neuts
	7% eos
	3% basos
	10% monos
	55% lymphs

Is this normal? Do you have too few neutrophils? too many lymphocytes, eosinophils and basophils?

The myeloid, monocytic, and lymphocytic leukocytes of the peripheral blood originate, and for the most part, mature in the bone marrow.

In the marrow microenvironment numerous hematopoietic growth factors influence the selection and stimulation of stem cells to the production of leukocytes.