Lymphocytosis

Introduction

Lymphocytosis refers to an increase in the number of circulating lymphocytes in the blood. The major causes of lymphocytosis are:

• Several types of neoplasia may result in increases in the number of blood lymphocytes, including:
  • Stage V (leukaemic) lymphoma
  • Acute lymphoblastic and chronic lymphocytic leukaemia
  • Bovine Leukosis caused by bovine leukaemia virus (BoLV)
• Stress may result in the release of lymphocytes into the circulation, especially in young horses and cats. This response is mediated by the release of adrenaline from the adrenal medulla and it should not be confused with the stress leucogram produced by the release of the glucocorticoid hormones cortisol from the adrenal cortex.
• Some cases of chronic infection may be associated with lymphocytosis but reactive hyperplasia in associated lymph nodes may not correlate with the numbers of circulating lymphocytes.
• Physiological lymphocytosis is common in young animals of many species. In growing pigs, this may be associated with Mycoplasma infection.
• In premature or dysmature equine neonates, the normal neutrophil: lymphocyte ratio of approximately 2: 1 may be reversed to produce an apparent lymphocytosis. Such animals will also have an increased mean corpuscular volume (MCV) of their red blood cells.
• Hypoadrenocorticism may result in lymphocytosis and/or eosinophilia.

Lymphocytosis Learning Resources
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