The immune system includes any part of a cat’s body that protects it against infection. The body’s surface—the skin and mucous membranes—acts as a physical barrier to disease-causing germs (pathogens).
The strong acid in the cat’s stomach kills many of the germs that enter through the mouth or nose. Those that do manage to enter the body through a cut or tear face attack from the major component of the immune system: white blood cells, also known as leucocytes.
Millions of white blood cells, made by bone marrow, are found in the bloodstream and in the lymphatic system. This system is a network of vessels throughout the body that collect and drain a watery fluid called lymph from body organs.
Its vessels are dotted with small nodes packed with white blood cells. The nodes filter the lymph, and the white blood cells attack any trapped germs. The tonsils, thymus, spleen, and the lining of the small intestine are also part of the lymphatic system.
There are several types of white blood cell, each with a different role. They identify and attack pathogens, such as bacteria, viruses, fungi, protozoa, and parasites, as well as any harmful chemicals (toxins) they may produce.
White blood cells include:
– Neutrophils, which engulf and destroy bacteria and fungi at the site of the infection, such as a wound.
– T-cells, or T-lymphocytes, which have a variety of roles, including the regulation of B-lymphocytes and attacking virus-infected and tumor cells.
– B-lymphocytes, which produce proteins called antibodies that attach to and neutralize pathogens.
– Eosinophils, which target parasites and are also involved in allergic responses.
– Macrophages, which engulf and digest the pathogens marked out by other white blood cells.
Cats are at risk of infection from bacteria, viruses, and other infectious agents. To keep healthy they have a powerful immune system with protective white blood cells that recognize “foreign” invaders, quickly destroying them before they multiply.
Sometimes the immune system reacts inappropriately, causing allergies or autoimmune disorders. The immune system weakens as a cat ages.