Getting to Know Your Cat’s Body: LYMPHATIC AND IMMUNE SYSTEMS

The Classic Comprehensive Handbook of Cat Care


The lymphatic system consists of lymph nodes and a network of thin- walled, permeable lymph channels and collecting ducts distributed throughout the body and associated with localized lymph nodes. The lymph nodes filter the tissue fluids (which constantly bathe all body cells), removing foreign particulate matter and returning the fluids and blood cells they may contain to the general circulation via lymphatic channels that eventually empty into the great veins associated with the heart. Through its immune functions the lymphatic system also provides a way for the body to detect, identify, and destroy foreign material that invades it.

Simply put, the immune system processes foreign materials such as viruses, bacteria, other microbes, and environmental proteins through specialized white blood cells. Neutrophils, produced in the bone marrow and normally suspended in blood, can migrate quickly to the site of an infection to destroy and engulf some foreign materials and microbes.

Macrophages can engulf invaders not destroyed by neutrophils and can signal other cells in the immune system to respond. Two types of lymphocytes, T cells and B cells, play central roles in the immune process. These cells are distributed throughout the body but are found aggregated in the lymph nodes, bone marrow, and spleen.

T cells arise in the fetal bone marrow and must be processed in the thymus gland to become functional. They play a major role in the body’s ability to recognize itself and are, therefore, important in eliminating cancer cells and infectious agents. They also play a major role in graft rejection and in certain allergic responses. T cells elaborate complex protein substances that act on other cells in the immune process in a wide range of biological activities.

B cells mature in the bone marrow, which serves as a continuing source of this kind of lymphocyte throughout life. B cells are activated to produce proteins called antibodies that are very specific for the antigens (proteins identified as foreign) that provoke them. B cells can produce their antibodies only with the aid of specific T cells (called helper T cells) acting together with specific stimulated macrophages that present the antigen to both the T and B lymphocytes. As time passes, the immunostimulated B lymphocytes differentiate into plasma cells, which are short-lived but capable of producing large quantities of antibodies, and long-lived memory B cells. These memory cells retain the ability to respond rapidly with antibody production specific for the same invader should reexposure occur.

Although you cannot detect most parts of the immune system during your physical exam, its normal daily functioning is vital to your cat’s health. For information on how the immune system affects preventive vaccination of your pet.