All About The Equine Immune System

The horse's immune system's primary function is to defend the body against harmful bacteria, viruses, fungi, foreign proteins and parasites, which can be collectively referred to as antigens.

Immune systems are hugely complicated, which makes them very effective at repelling antigens. However, factors such as old age, bad nutrition and stress can all affect how well the immune system functions. In horses that are very old or very young, malnourished or under stress, there is a higher risk of disease, as the immune system is not functioning as well as it should be.

Horses that are stressed are usually those experiencing changes in feeding, management and social interactions. All of these factors may inadvertently cause a horse to be stressed, whether it is a competition or a leisure horse. This can make them more susceptible to disease-causing antigens.

So, how does a horse's immune system function?

• Firstly, physical barriers will help to stop antigens from entering the body. The physical barriers are the skin, cornea (in the eyes), the membranes lining respiratory passages, the digestive tract, and the urinary and reproductive tracts. As long as these barriers are functional and healthy, they will stop many disease-causing invaders from entering the body.
• Secondly, horses have non-specific immunity. If antigens do get past the physical barriers, chemical messengers will alert the immune system and the inflammatory response, which, on a horse, we will sometimes see as heat and swelling. This type of immunity is present from a foal's birth, helped by the mare's own immune system. Non-specific immunity cannot differentiate between antigens, but can recognise that they are foreign and so a response is initiated. Which blood cells will move to the area of invasion and kill invaders while removing damaged cells.

Specific immunity can differentiate between invaders, and it can 'learn' between different antigens and the best way to attack. This means that if they invade again, the immune system will react more quickly and efficiently - this is the concept behind vaccines! This also involves white blood cells, but a specific type called lymphocytes.