White blood cells, also called leukocytes, are an important part of the body's defense against disease. In response to infection the WBC count typically increases. Therefore, measure of the WBC count can give an indication if there is infection or inflammation occurring. Increases in WBCs can also occur in some cancer conditions such as leukemia. A bird under stress may have a WBC that is doubled, but greater elevations are a definite indication of disease. Reduced numbers of WBCs can occur due to bone marrow disease, severe acute disease and other conditions.
At this time, there is no automated system as is used with humans or dogs and cats that has proven effective for determining avian white blood cell counts. The complicating factor is that birds have nucleated RBCs in addition to nucleated WBCs. Most automated systems count nucleated cells, which are WBCs, as humans, dogs, cats, and other mammals have non-nucleated RBCs. The nucleated avian RBCs interfere with most automated counting methods. A new system that shows promise is laser flow cytometry which may be able to perform automated WBC counts in birds. Veterinarians who do not feel comfortable performing hematology procedures send the blood samples to one of the commercial laboratories that perform avian clinical pathology.
There are different varieties of WBCs and changes in their numbers can indicate particular disease conditions. The cells include heterophils (equivalent to the mammalian neutrophil or PMN), lymphocytes, monocytes, eosinophils, and basophils. A differential count can then be performed to evaluate the percentages of each type of cell. The normal distribution of leukocytes is approximately 50% heterophils and 50% lymphocytes with small percentages of the other cells, but different factors will change the distribution. Some species respond to stress with a lymphocytosis (increased lymphocytes), while others do so with a heterophilia (increased heterophils). Allergic or parasitic conditions may show increased numbers of monocytes (monocytosis). Monocytosis can occur in cases of chlamydiosis (psittacosis).
Reference: "Understanding Avian Laboratory Tests" by Peter S. Sakas DVM, MS