Some species, such as cockatiels and Amazon parrots, are naturally lymphocytic, which means more lymphs than hets are seen normally. Lymphocytes are a very important part of the bird s immune system. They decrease in numbers in acute viral infections. In a disease state, lymphocytes will become reactive. A few reactive lymphs on a blood smear is normal but if many are reactive then this is a significant finding that the body is responding to an infection of some sort.
Reactive lymphs synthesize antibodies and other agents involved in the host immune system.
Birds with a marked lymphocytosis in which most or all of the lymphocytes appear abnormal or immature is consistent with leukemia. These are the largest WBCs found in avian blood, and they are very similar in appearance to lymphocytes. Changes in cell morphology are not significant. The evaluation of the WBC count is most useful in the assessment of the progress of an avian patient. For example, a bird with an initial leukocytosis, increased heterophils which appear toxic, and reactive lymphocytes would be showing a favorable response to therapy if the total WBC and heterophil counts were decreasing, the toxic heterophils had disappeared, and the reactive lymphocytes returned to normal.