Changes in leukocytes in blood are incredibly helpful in identifying underlying physiologic (e.g. stress) and systemic disease processes, such as inflammation. These changes include morphologic features of leukocytes (which are covered in a separate section under hematology) as well as changes in numbers. For some leukocytes, only increased numbers are clinically relevant, whereas decreased numbers have no diagnostic meaning (e.g. monocytes). Changes in numbers are identified when the values exceed reference intervals for that species. There are many species in which reference intervals are not available. In these cases, the clinician can refer to published data for guidelines on intervals and also use common sense in interpreting the lab data.
In this section, information will be provided on the following:
- Individual leukocytes: Basics on the different leukocytes and the diagnostic relevance of changes in their number. Information is also given here on leukocytes that are only seen in blood in sick animals, including histiocytes and mast cells.
- Leukocyte patterns: How changes in several leukocytes together give us information about physiologic and pathologic processes, including a stress leukogram, physiologic leukocytosis and inflammatory leukogram.
- We provide useful tips and a table on differentiating different types of leukogram patterns
- We also provide some guidelines in table format on how we interpret changes in the proportions of mature/segmented or immature/band neutrophils in relation to each other with applicable terminology
- Brief information will also be given on leukemia, however this will also be covered in a separate section on leukemia.
Please also refer to the atlas for images of different leukocytes in blood.
Some definitions may be useful:
- Granulocytes: This refers to leukocytes that have cytoplasmic granules that are specific to the leukocyte. These are secondary granules that are produced at the myelocyte stage of maturation. These cells also have segmented nuclei, distinguishing them from mononuclear cells in blood.
- Neutrophils: These have pink granules. In some species, e.g. cattle, granules are more evident than in others, e.g. dogs, where the neutrophil cytoplasm looks colorless.
- Eosinophils: These have red granules. In some species, e.g. felidae, the granules are rod-shaped. In most other species, the granules are round.
- Basophils: These have purple granules. In dogs and cats, basophil granules are lavender (very light purple) and difficult to see, whereas in horses, camelids and cattle, the granules are purple (and resemble mast cell granules).
- Mononuclear cells: These are cells that have non-segmented nuclei.
- Lymphocytes: These generally have round nuclei with very high nuclear to cytoplasmic ratios (scant amount of cytoplasm).
- Monocytes: These have more pleomorphic nuclei, that can be lobulated (but the lobules are “fatter” than those of granulocytes).
- Left shift: This indicates that there are immature neutrophils in circulation. Immature neutrophils are identified by the shape of their nucleus which is not segmented (non-segmented). Immature neutrophils (in order of decreasing maturity, i.e. most mature to least mature) include band neutrophils, metamyelocytes and myelocytes.
- Degenerative left shift: This indicates that there are more immature than mature neutrophils, regardless of the total white blood cell or neutrophil count. It indicates severe inflammation (and is usually due to bacterial sepsis).
- Regenerative left shift: This indicates that there is a neutrophilia with increased numbers of band neutrophils (compared to reference intervals). This term indicates that the bone marrow is responding to a source of inflammation, but we don’t use this term at Cornell University.
- For more information on how we interpret changes in the proportions of mature/segmented to immature/band or other forms of neutrophils, please refer to leukogram patterns for useful tips and a guideline table.
- Toxic change: This indicates inflammation and is only evident in the neutrophil lineage. Toxic change refers to features of immaturity in neutrophils, since there is accelerated maturation and release from bone marrow (as part of the inflammatory response). It does not refer to the toxic effect of bacteria or their toxins on these cells. For more information on toxic change, refer to morphologic changes in the hematology section of this site.