White blood cell morphologic features are less species-dependent than red blood cells, although there are some species peculiarities (e.g. neutrophils in healthy cats can have low numbers of small Döhle bodies). The same basic principles are followed for identifying leukocytes in different animal species, with the exception of exotics and small mammals (which have heterophils instead of neutrophils). Changes in the morphologic features of white blood cells can give us clues as to underlying diseases. For instance the presence of toxic change indicates the presence of inflammatory cytokines with accelerated maturation and release from the bone marrow. This indicates an inflammatory leukogram and usually accompanies the presence of immature neutrophils in blood (i.e. a left shift is present). Other changes are of less diagnostic relevance, e.g. reactive lymphocytes indicates non-specific antigenic stimulation.
In this section, we will cover:
- Normal leukocytes: This covers the normal leukocytes seen in blood, including granulocytes (neutrophils, eosinophils, basophils) and mononuclear cells (monocytes, lymphocytes). Clues on how to differentiate between leukocyte and leukocyte mimics (nRBC and lymphocyte) are also given here. Species differences in normal granulocytes and mononuclear cells will be provided.
- Abnormal features:
- Abnormal leukocytes: Cells that are not typically seen in blood but can be under inflammatory or neoplastic conditions, e.g. mast cells, histiocytes, blasts (in a leukemia).
- Left shift: Presence of immature neutrophils in blood. This indicates an inflammatory leukogram under most situations.
- Toxic change in neutrophils: This indicates neutrophil immaturity (accelerated release in response to inflammation) and also indicates and inflammatory leukogram in most situations.
- Reactive lymphocytes: These indicate antigenic stimulation. We have also included a category of “big blue cells” (which are large mononuclear cells found in blood, which could be reactive or neoplastic) along with strategies to decide what they most likely are.
- Nuclear changes: Hyper- or hyposegmentation of neutrophils.
- Cytoplasmic changes: Presence of abnormal granules, hemosiderin (sideroleukocyte) or vacuoles within leukocytes can be a marker of underlying disease.
- Dysplasia: Abnormal features of WBC secondary to accelerated or abnormal maturation of cells in marrow.
- Infectious agents: Some infectious agents, including fungi (Histoplasma), protozoa (Toxoplasma, Sarcocystis), bacteria (Ehrlichia, Anaplasma), can be seen within leukocytes in blood, facilitating diagnosis of these diseases.