Even if all body systems work properly, it is impossible to escape the insult of pathogens, carcinogens and other organisms and substances that attack the body. The body will fail, unless adequate defenses are mounted to "protect" against such invasion. Through the cardiovascular system and lymphatic system, the immune system has access to all parts of the body. In the simplest terms, the immune function is based on the activities of innate and adoptive immune mechanisms.
Innate Immune Mechanisms --The first line of defense is the series of barriers to pathogens: the skin, sebaceous (skin gland) secretions, mucus, cilia (fine hairs lining the windpipe), stomach's acid, various beneficial flora/fauna, and the bacteriocide, lysozyme, found in tears and other body secretions. If an invading organism gets past these rather potent defenses it may be delayed or destroyed by natural killer cells or by phagocytes (scavenger cells). In the older dog, loss of barrier integrity, such as with chronic skin conditions, leaves an opening to the outer world, and the ability to fend of pathogens with innate immune mechanisms is reduced.
Adaptive Immune Mechanism --The ability to store information about past exposure to pathogens, to remember what worked against it, and how to quickly produce and transport these cells to the site of infection, is a critical part of the adaptive immune mechanism. Lymphocyte (white blood cells) B cells, helper T cells and killer T cells are quickly produced in great quantities, and for most infections effectively target specific invader pathogens. With age comes a reduction in the size of the thymus gland and cell-mediated immunity is diminished in older animals.
In older dogs, there is also a diminished ability to recognize "self" and the immune system may produce antibodies or most specifically autoantibodies that target the body's own cells. Such autoimmune response is seen in canine rheumatoid arthritis, polyarthritis, myasthenia gravis, polymyostitis, autoimmune hemolytic anemic, immune-mediated thrombocytopenia, autoimmune thyroiditis, systemic lupus erythematosus, and a wide range of autoimmune diseases of the skin such as the various forms of pemphigus. Diet restriction throughout life slows down the reduction in size of the thymus gland and may make it possible to maintain immune system efficiency for a longer period of time, resulting in some measure of life extension.