Horses, like all mammals, have a highly interactive multi-layered immune system composed of organs, tissues, cells and chemicals designed to protect the body against the continuous everyday exposure to billions of microbes (viruses, bacteria, protozoa) and allergens, and to protect and restore body organs, muscles, joints and bones against various injuries ranging from pulled muscles and minor cuts to more traumatic events.
The immune system is the body’s department of defense, and its army of rapid attack and complementary response teams forms the front lines (the innate immune system). The innate system consists of physical barriers to infection, immune cells and other complementary proteins and cytokines. Protective physical barriers include the skin, saliva, tears and the mucus secretions of the intestinal, respiratory and reproductive systems.
The immune system stockpiles a huge arsenal of cells, some of which take on all intruders (the innate system), and others that are trained on highly specific targets (the acquired immune system). Sometimes they work by direct physical contact, and sometimes they release chemical messengers called cytokines.
Both good and bad microorganisms constantly come in contact with the body’s immune system. A normal immune system is always working to distinguish between which are pathogens, or bad "invaders," and which are good "allies" that are useful for the body. The word "microbes," though it is synonymous with "microorganisms," sometimes refers to the bad or harmful invaders, while the term "probiotics" often designates the beneficial ones.
The good allies assist in body defense systems and aid digestion. The immune system has a “high-tech” detection system designed to sort out the good and bad microbes. The good microbes are “scanned” and let through so they can become allies. The bad ones are either killed or simply eaten by the rapid response team defenders. The longer-term, or acquired, immune system remembers which microbes are good and which are bad, and stores this information in its "database" so the next time the immune system is challenged it can recognize the invaders and respond with a more specialized rapid response team (antibodies). These antibodies are produced by plasma cells to identify and neutralize pathogens, or harmful microbes.
Just a few of each kind of these antibodies are stored in the system, as it only takes a few to recognize millions of possible enemies. When an invader first appears, the few immune cells that recognize it respond by multiplying into a full-scale army of cells. After their job is done, most of the cells fade away, leaving a few sentries behind to watch for future attacks.
The immune system is designed to respond to a stress such as disease or injury in an efficient manner to help the body recover quickly and then return to a normal state of balance. If, as in the case of a chronic illness, the body is unable to recover quickly, the immune system remains activated too long, causing loss of body condition.
When a horse becomes sick or injured, the immune system sends communications to speed up the metabolism rate, cause swelling (inflammation) and increase body temperature (fever) to kill the invaders. Short-term inflammation, though it causes pain, can be beneficial, as it triggers undamaged cells in a damaged area of the body to swell, which can help protect them against further damage by invading pathogens.
However, if this activated immune response and the discomfort associated with inflammation lasts too long, it can be harmful because it requires a lot of nutrients (protein and energy) to support the activated immune response. This takes away from the pool of nutrients needed to support growth and other normal body functions. Further depleting the nutrient pool is the lack of appetite associated with illness. So while the activated immune system is making more demands, the horse is consuming fewer nutrients.
In a chronically activated immune system, body tissues such as fat, muscle, and even bone are eventually broken down to provide supplemental nutrients to support and maintain the activated immune response, as well as regular bodily functions.
This powerful and efficient system, which is designed to protect and defend, can do just the opposite if it remains out of balance.
Recent research demonstrates that orally-dosed, serum-based bioactive proteins found in products such as LIFELINE Equine Performance Supplements have multiple effects benefitting the immune system reaction in different body systems. In animals, including horses, orally-dosed bioactive proteins have been shown to improve intestinal barrier function. In humans, bioactive proteins reduce intestinal permeability and minimize chronic immune stimulation in patients with Crohn’s disease and IBD.
Author Pam Maley is an educator who, with her husband, co-owned and -managed a Thoroughbred broodmare farm in Lexington, Kentucky, where she was closely involved with all aspects of equine life. An avid former foxhunter, her journey with horses continues as a writer and editor with EquestriSol. This is the first part of her three-part series detailing Immune System Basics. Part II will examine common stressors for horses and the immune system’s reaction to that stress. Part III will detail the immune system reaction in the horse’s muscular, digestive and respiratory systems.