White Blood Cells.
White blood cells, or leukocytes, are the major components of the humoral immune system and are the first responders to systemic infection, disease and invasion of the body. While the red blood cell indices were important to us in terms of oxygen carrying capacity and delivering a prognosis with several different disease processes, the white blood cell profiles are invaluable to us in determining what types of diseases we might be dealing with and how serious an illness we are looking at.
Here I will lay out the major leukocytes, what we are looking for and what they tell us. This is by no means an extensive look at the white blood cells but will serve as an adequate overview to give you an idea of what we are looking for and hopefully in turn make you a more informed owner.
Neutrophils: These are the first responder cells and the stars of the white blood cell count. They both initiate and modify inflammation by secreting pro-inflammatory proteins. They also release cytotoxic proteins to destroy invading cells and phagocytize or eat micro-biotic invaders. They are the most prominent white blood cell population and will increase in response to stress, pain, steroid use, infection and inflammation. The numbers will decrease in response to septicemia, chemotherapy or some immune mediated conditions. Whenever we see a change in neutrophils in one direction or the other we look at a slide of blood under the microscope for a number of reasons. In truth, we almost always look at a blood film anyway and really should - in best practice - always compare our blood work to a visual blood film. When we see toxic changes or a degradation of our neutrophil population we start to think about severe diseases. If we see a lot of immature neutrophils we have to go back and look at our total white blood cell population. If the number of immature neutrophils outnumbers the mature neutrophils, we call this a "left shift". If the total white blood cell count is high and there is a left shift it indicates severe inflammation or necrosis but the bone marrow is responding and the prognosis is typically still good. If there is a left shift and the total white blood cell count is low or normal this means the bone marrow is unable to respond and that the patient has a systemic toxemia; this carries a guarded to bad prognosis.
Lymphocytes: These are the second most prominent cells and play a strong supporting role to the neutrophils. Lymphocytes are made up of two major groups, the T cells which are the responders (these are involved in the cell mediated immune system) and the B cells (the producers of antibodies in response to disease). The T cells are the ones who respond to foreign antigens, and the B cells are the ones who make the antibodies to inform other cells that the antigen is a bad guy. Lymphocytes will be increased naturally in young cats and in animals that are stressed or trembling. They will increase in response to chronic inflammation, some viruses and of course in leukemia and lymphoma. We see decreases in the number of lymphocytes in response to steroid therapy or other immunosuppressive medications. Some viruses will decrease lymphocyte numbers and there are certain specific conditions that will either sequester or destroy lymphocytes. Chylothorax and protein losing enteropathy are the most common ones I can think of.
Eosinophils: These are pretty rare cells. In fact, when I'm looking at a blood film I only notice when I don't see eosinophils because I like to see them. They are pretty cells with blue-purple nucleus and red-pink granules that are round in dogs but rod shaped in cats. Eosinpohils are involved in acute inflammation, allergies and they help to control parasites. Increased numbers typically indicate parasitism or allergic/inflammatory conditions that involve IgE like asthma. It's very difficult to determine when low numbers of eosinophils are present because their numbers are so low normally.
Basophils: Another rare cell line. In fact we rarely see basophils when we are looking at a blood film. These are also pretty cells to see with dark purple granules. They are often increased along with Eosinophils in conditions that involve IgE and their function is largely unknown. They are thought to play a role in the initiation and modification of inflammation.
Monocytes: Another rare cell line these cells remain in circulation for less than one day before leaving to become tissue macrophages, cells that ingest foreign material or cellular debris to keep things tidy and limit reactivity. When they do remain in circulation or are active within the blood stream they are heavily involved in chronic inflammation but may increase in numbers as a response to acute inflammation. Decreased numbers of monocytes are difficult to determine because of naturally low numbers and the relatively short amount of time spent in circulation.
When we are looking at a differentiated white blood cell count, we get back the numbers in two formats, an absolute count and each cell line's percentage of the population. The absolute count is important in determining who is prevalent and this information will give us insight into the problems that encouraged us to run the blood work in the first place. The percent population is a good way to understand how the cell lines are responding to the issue.
The final cell line involved in a complete blood count (CBC) is the platelet. Platelets are small non-nucleated cells that are involved in blood clotting in response to injury or disease. They also release factors that will initiate the subsequent steps in coagulation in response to bleeding whether internal or external. Increased numbers of platelets can indicate some disease processes; we will generally rely on other indicators to take our diagnostic plan in a specific direction. Low numbers of platelets can indicate bleeding going on somewhere in body. Decreased platelets can also be due to medications, infections or cancers that affect the bone marrow. Whenever we see low numbers of platelets on our lab work we always, always, always make a blood smear and look for signs that there are adequate platelet numbers and the platelets we are seeing look healthy. This is especially important if the patient is scheduled for some sort of surgical procedure where there may be significant bleeding.
This week combined with the last blog about red blood cells completes our Complete Blood Count or CBC section of our lab work. This information is meant to help you understand what your veterinarian is explaining to you as they go over lab work. Any further explanation can be discussed with you by your vet.
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