The circulating white blood cells include the granulocytes, the monocytes, and the lymphocytes. From a functional perspective, these cells may be divided into two systems: the phagocytes and the immunocytes. The phagocytes include the granulocytes and the monocyte/macrophage continuum. The immunocytes include the B and T lymphocytes.

The phagocytic system is the first line of defense against infectious disease and may be thought of as the non-specific immune system. The immunocytic system responds to specific antigens by producing antibodies and lymphokines and is termed the specific immune system. The two systems are linked through the functions of the monocyte/macrophage continuum.

Clearly, the peripheral blood leukocytes play a central role in the animal's response to disease. Understanding this role and being able to interpret abnormal peripheral leukocyte responses will provide the clinician with significant diagnostic, therapeutic, and prognostic insights. The following paragraphs detail the normal structure and function of the various circulating leukocytes and briefly address the interpretation of abnormal leukocyte morphology and number. Evaluation of the phagocytic system is emphasized since changes in this system are the most significant to clinical evaluation.

Normal granulocyte structure and function

Granulocytes of the dog and cat include neutrophils, eosinophils, and basophils. In terms of circulating numbers, the neutrophil is by far the most prevalent. The three granulocytes are distinguished on the basis of their cytomorphology; they are also distinct functionally and changes in circulating cell numbers of the three granulocytes are interpreted quite differently.

In peripheral blood films, neutrophils are round cells measuring 12-15μ with pale pink granular cytoplasm and nuclei with condensed chromatin forming nuclear lobes connected by thin bands of chromatin. Nuclear morphology of neutrophils is classically described as segmented.

From a functional perspective, the neutrophil is a fully armed phagocyte which can be immediately mobilized when microorganisms, particularly bacteria, invade the animal. Circulating neutrophils are drawn from blood into the tissues at sites of inflammation where there is a high concentration of chemoattractants (chemotactic stimuli). This process of directed movement of phagocytes along a chemotactic gradient is known as chemotaxis. Chemoattractants for neutrophils include bacterial lipopolysaccharide, complement fragments, immune complexes, etc.

When neutrophils arrive at a site of inflammation, they adhere to those particles for which they have receptors on their surfaces. In particular, neutrophils have surface receptors for things such as immunoglobulins and complement fragments; when bacteria are coated by such molecules (called opsonins), adherence is facilitated.

Adherence is followed by phagosome formation. In this process, the cell membrane of the phagocyte literally flows around the adhered bacterium, resulting in internalization of the organism within a membrane-bound vacuole.

Phagosome membranes fuse with membranes of neutrophil cytoplasmic granules, thereby releasing granule content in close proximity to the internalized bacterium yet separated from the neutrophil cytoplasm. Neutrophil granules are of two types. Specific granules contain lysozyme, lactoferrin, collagenase and plasminogen activators. Azurophilic (primary) granules are lysosomes which contain acid hydrolases, myeloperoxidase, lysozyme and cationic proteins. Collectively granule content of the neutrophil is responsible for both bacterial killing and digestion. Altogether, the process of bacterial identification, entrapment, killing, and digestion is the process of phagocytosis.

Eosinophils are granulocytes which contain orange (eosinophilic) cytoplasmic granules. Granules stain with eosin because of their content of basic mucopolysaccharide (including antihistamines) and major basic protein. Granule appearance in eosinophils is characteristic of the species. Dog eosinophils have variable numbers of variably-sized round granules. Cytoplasm of canine eosinophils is often vacuolated. Feline eosinophils contain large numbers of uniform rod-shaped granules. Like neutrophils, both canine and feline eosinophils have segmented nuclei, but, in general, eosinophil nuclei stain less intensely than neutrophil nuclei.

Like neutrophils, eosinophils are phagocytes capable of ingesting, killing, and digesting bacterial organisms. However, eosinophils are less effective at bacterial phagocytosis than neutrophils and this is not their primary function. Rather, they are involved primarily in the IgE mediated destruction of metazoan parasites, in controlling allergic reactions and as moderators of acute inflammation (due to their antihistamine content). Whenever circulating eosinophilia is observed, systemic allergic reaction should be suspected. It is emphasized that intestinal parasitism is not a cause of systemic allergy and therefore should not be accepted as a cause of peripheral eosinophilia.

Basophils are named for their basophilic (blue to purple) cytoplasmic granules which acquire their stain affinity from their content of sulfated (acid) mucopolysaccharides (histamine, heparin, serotonin). In reality, in canine and feline basophils these granules can be quite sparse, making these cells difficult to identify. Basophils are generally slightly larger than neutrophils, the nucleus is less hyperchromic and the background cytoplasm is more basophilic.

Functionally, basophils are not phagocytes. For years, their role has remained somewhat obscure. Current theories are that basophils (and their tissue counterparts the mast cells) are initiators of the acute inflammatory response and in this function they are opposed by the moderating effects of the eosinophil (see above). Additionally, basophils release their histamine when stimulated by reaginic antibodies (Ig E) and therefore are important as facilitators of systemic allergic reactions. When circulating basophilia is seen, systemic allergy should always be suspected.