A healthy immune system turns on and off as necessary. If the system doesn't turn on properly a pet has a poor immune response and is 'immunosuppressed'. Both humans and cats are affected by virus diseases that suppress the immune response (HIV in humans, FIV and FeLV in cats). There are no known equivalent infectious causes of immunosuppression in dogs. If the immune system is over-sensitive or doesn't turn off properly, allergy or auto-immune disease occurs. We diagnose both of these conditions with increasing frequency. This may be because we are more knowledgeable about allergy and auto-immune disease but it may also be because, as in humans the incidence of canine and feline conditions involving an over-active immune system increased in the last half of the twentieth century.
How The Immune System Works
White blood cells are the attack soldiers of the immune system. Their function is to guard and protect the body, to maintain homeostasis. An efficient immune system recognises and destroys internal dangers like cancers cells and external pathogens such as viruses, mycoplasma, bacteria and yeast. Bone marrow, the thymus gland, lymph nodes and the spleen are the visible internal elements of the immune system but in fact the skin and the lining of the gut are its largest components. These are the front lines of defence. Immune system cells called helper T-cells prompt other white blood cells called B-lymphocytes to produce antibodies to label dangerous microbes. Suppressor T-cells instruct the B-lymphocytes to turn off antibody production when a job is completed. Appropriately named 'natural killer cells' attack and destroy tumour cells and virus particles. Other lymphocytes called memory T-cells patrol the body recognising organisms they have met in the past, rapidly mobilising attack teams. And, like sanitation workers, macrophages, the 'big eaters', clean up the mess on the battlefield, devouring microbes and debris.