Vaccines are designed based on the specific nature of an antibody response to an antigen. In other words, the antibody will work only against the antigen that stimulated its production. A “booster” creates more antibodies, a quicker response, and longer protection.

The equine immune system, which is designed to protect a horse from invading pathogens, is extremely complex. When everything is functioning in synchrony, the system works well. The problem is that many things can compromise the immune system, and when that happens, the horse is at an increased risk of developing disease. Often one (or more) of three key elements are at the root of the problem when the immune system becomes compromised, says Glen Gamble, DVM, of Riverton, Wyo. They are stress, nutrition, and age.

When a horse is stressed, lacking in proper nutrients, or old, he says, the immune system can’t function appropriately and pathogens are able to breach the defensive lines. But before we can understand how that works, we must first understand the immune system.

One of the most interesting descriptions of an animal’s immune system is provided by Ian Tizard, PhD, BSc, BVMS, MRCVS, who authored the book, Veterinary Immunology, An Introduction.

“In some ways,” he wrote, “the immune system may be compared to a totalitarian state in which foreigners are expelled and citizens who behave themselves are tolerated, but those who ‘deviate’ are eliminated. While this analogy must not be pursued too far, it is apparent that such regimes possess a number of characteristic features. These include border defenses and a police force that keeps the population under surveillance and promptly eliminates dissidents. Organizations of this type also tend to develop a pass system, so that foreigners not possessing certain identifying features are promptly identified and dealt with.”

Fighting the