Your Dog's Immune System
Your dog has a wonderful defense against diseases and health problems — the immune system, which constantly goes to battle to fight off invaders. The immune system uses several types of cells in the bloodstream to fight off bacterial and viral invaders, including lymphocytes, neutrophils, and macrophages. These cells develop within the bone marrow, spleen, and lymph nodes.
There are two main groups of defender cells, or lymphocytes — B cells and T cells. The B cells produce antibodies against bacteria. B cells can be exposed to antigens through plasma cells or on their own, and they then develop specific antibodies against those antigens. T cells often work via the thymus (tissue in the chest that helps these lymphocytes develop) and are active against viruses and delayed hypersensitivity.
The immune system also includes secretions known as immunoglobulins that act as antibodies and respond to infections. The primary immunoglobulins are Ig A, which acts near mucosal surfaces such as the moist linings of the mouth and the nose to prevent bacteria and viruses from entering the body; Ig E, which acts along the lining of the respiratory and intestinal tracts to keep invaders out; and Ig G, which can be found throughout the body fighting infections. These immunoglobulins can cause problems, too — they can become sensitized to your dog's own tissues or react against foreign tissue such as a kidney transplant.
Your dog also has cells that actively attack and ideally consume and destroy bacteria and viruses. These cells include the neutrophils (in the category of white blood cells), the macrophages, and the mast cells. Neutrophils and macrophages may actually eat bacteria and viruses. Mast cells release many immune mediators such as histamine. These mediator substances can make your dog miserable even while her body is trying to help. Think of the symptoms that lead you to take antihistamine medications!
When your veterinarian checks a complete blood count on your dog, she is looking at white blood cell counts and a differential. The white blood cell count tells her how many infection-fighting cells are in your dog's bloodstream. The differential, determined by looking at the cells on a microscope slide, gives her an idea of how many cells there are of each type of white blood cell — for example, how many neutrophils, how many lymphocytes, etc. These measurements can guide your veterinarian in diagnosing the cause of your dog's illness and planning an effective treatment.
Autoimmune problems such as autoimmune hemolytic anemia and certain skin problems develop when the immune system cells fail to recognize your dog's own cells. The immune cells attack the dog's other cells because they appear to be foreign invaders. These can be difficult conditions to treat.
Your Dog's Natural Defenses
Your dog has a number of defense mechanisms in addition to the immune system itself. Virtually all of your dog's systems have some built-in safeguards and alarms. Together with the immune system, they work to keep your dog healthy and happy.
Your dog's skin is a barrier to bacteria, viruses, fungal infections, and parasites. Many of these disease agents can only get a foothold on your dog if the skin has been broken through trauma or internal disease reactions, such as allergies. Even your dog's coat acts as protection from these outside invaders.
Along the respiratory tract are cells that trap and catch dust and other debris, including bacteria and viruses that are trying to make their way into the lungs. Coughing and sneezing remove many of those infectious agents and irritants. Eyes produce tears that wash invaders away. Problems that reach the stomach may be actively removed by vomiting. The gastrointestinal tract secretes mucus to help protect it from caustic substances and may speed up passage to remove toxic wastes quickly via diarrhea.