Reading a CBC, Lymphocytes

Lymphocytes may be counted by machines or by a technician doing a manual differential or "manual diff".  This involves taking a drop of blood and making a smear on a slide.  It is allowed to dry, then stained with Diff Quick stain.  Once the stain has dried the slide is viewed under a microscope and evaluated for red and white blood cell morphology as well as a count of 100 white blood cells to determine what percent of cells are of each WBC type including lymphocytes, which are the most common white blood cells in cattle.

The number of lymphocytes may be expressed as a percent or as an absolute number which is expressed as some number per microliter.  If you have a choice it is best to work with the absolute number since, with abnormal WBC counts, the number expressed as a percent can be misleading.

Below is a photo of 2 normal lymphocytes:

Lymphocytes include the cells responsible for both humoral and cell-mediated immunity.  Sometimes they may appear with blue cytoplasm, these are reactive forms which are likely B cells.

Increased lymphocytes or "lymphocytosis" is caused by:
-Excitement: this is common in cats.  If they become overly excited during the blood draw all of the cells that are normally loosely attached to the endothelial lining of the blood vessels detach and end up in circulation where they can be drawn out and counted.  This means that the increase isn't "real" because there aren't really more cells being made, it's just that the ones that aren't usually counted because they stick to the blood vessel wall were counted even though in 30 minutes or so they'll go back to where they belong.  If this is the case you'll see increased numbers of neutrophils as well (neutrophilia).
-Leukemia


Decreased lymphocytes or "lymphocytopenia" is caused by:
-Stress: an increase in steroids which can also be due to the administration of steroids
-Acute viral infection: usually accompanied by beutropenia or a decrease in neutrophils as well.