As medical research teaches us more about the immune system, and as more and more autoimmune conditions are diagnosed in canines, especially purebreds, the issue of a dog's immunity is becoming a concern for would-be pet owners.
In all mammals, the immune system serves as the body's defense against disease. It's comprised of white blood cells, antibodies and various other chemicals the body uses in its war against infections and any substance it perceives as foreign to the host animal. It's designed to combat any of these cells that it identifies as "non-self," and it does this through chemical markers that are found on the surface of every cell in the animal's body. It's this natural combative design that causes a person's or animal's body to reject blood transfusions, organ transplants and skin grafts.
As with all systems, however, the immune system can fail, either by overachieving or underachieving. An immune system that mistakenly attacks its own cells can cause a wide variety of medical problems that are known collectively as autoimmune disorders. In other cases, a defective immune system is unable to protect the body adequately, leaving it vulnerable to infections and bacteria.
When puppies are first born, it's critical that they be allowed to nurse from their mother right away, in order to obtain colostrum. This is the mother's pre-milk, which is rich in antibodies and helps the puppy kick-start its own developing immune system. Puppies that do not get this milk are at high risk of death from any opportunistic infection. In fact, Fading Puppy Syndrome is a recognized condition in which puppies are born healthy but rapidly become ill, refuse to nurse and die. In general, puppies that are born to healthy, well-nourished mothers and then allowed to nurse for at least six weeks will fare best in terms of their overall immunity.
A side observation is that puppies raised in a "normal" environment with a healthy mother develop much stronger immune systems than those raised in artificially sterilized environments.
The sad truth is that while multiple theories are being researched, it's not yet known what causes autoimmune disorders in either humans or animals. Any breed of dog can be affected, and the potential causes of immune system deficiencies range from over vaccination to environmental pollutants, food preservatives and/or genetic factors. Few of these factors have been proven in human beings as yet, and fewer still in canines. Most autoimmune diseases are incurable and are managed with high doses of corticosteroids and other immunosuppressive drugs, to lower the dog's immune response. Dogs with these problems obviously should not be bred.