HOW DOES THE HORSE’S IMMUNE SYSTEM WORK?
Simplified diagram of the horse’s immune system with illustrated cells and pathogens.
The horse’s immune system plays a vital role in survival: it protects the body against pathogens (bacteria, viruses, parasites, fungi) while maintaining a delicate balance between defence and tolerance.
Understanding how it works helps to better assess equine health, anticipate vulnerabilities, and appreciate the importance of preventive care.
1. The two main components of the immune system
Like other mammals, horses have two complementary levels of defence.
1.1 Innate immunity: the first line of defence
It reacts quickly as soon as the horse encounters an infectious agent.
It includes:
- physical barriers: skin, mucous membranes, respiratory hairs, mucus, tear secretions;
- sentinel cells: macrophages, neutrophils, dendritic cells;
- immediate mechanisms: inflammation, fever, phagocytosis.
This immunity is non-specific: it recognises common patterns shared by many microbes.
1.2 Adaptive immunity: the targeted response
It develops more slowly but is precise and long-lasting.
It relies on:
- B lymphocytes, which produce antibodies,
- T lymphocytes, which destroy infected cells or coordinate the immune response.
This system enables the horse to acquire immune memory, essential for long-term protection, particularly after vaccination.
2. The key role of colostrum in foal immunity
Unlike humans, a foal receives almost no antibodies before birth: the equine placenta prevents them from passing through.
It is therefore born with an immature immune system and relies entirely on colostrum (first milk) to obtain:
- immunoglobulins (antibodies),
- growth factors,
- elements that support gut maturation and immune development.
A foal can only absorb these antibodies efficiently during the first 8 to 12 hours.
Insufficient intake can lead to severe infections in the first days of life.
3. The essential organs of the immune system
Several organs work together to protect the horse from pathogens.
3.1 Bone marrow
The site where immune cells (white blood cells, lymphocytes) are produced.
3.2 Thymus
Highly active in young horses, it “educates” T lymphocytes to prevent inappropriate immune reactions.
3.3 Spleen
Filters the blood, removes microbes, and activates certain lymphocytes.
3.4 Lymph nodes
Distributed throughout the body, they monitor tissues and trigger local immune responses.
3.5 Peyer’s patches and gut-associated lymphoid tissue
Crucial in horses, whose digestive system is large and sensitive.
4. Immune response: how the horse’s body reacts
When a pathogen enters the body, several steps follow.
4.1 Recognition
Innate immune cells detect the intruder through specialised receptors.
4.2 Signalling
Chemical messengers called cytokines trigger inflammation and attract immune cells to the infection site.
4.3 Action
- Macrophages and neutrophils directly destroy microbes.
- T lymphocytes identify infected cells.
- B lymphocytes produce specific antibodies.
4.4 Immune memory
If the pathogen is encountered again, the response is faster and more efficient.
5. Factors that weaken the equine immune system
Certain situations make the immune response more vulnerable:
- Stress (transport, change of stable, isolation)
- Nutritional deficiencies (vitamin E, selenium)
- Overtraining
- Very young or very old age
- Chronic diseases (Cushing/PPID, metabolic disorders)
- Internal parasites
- Poor environmental conditions (humidity, overcrowding, poor ventilation)
These factors increase the risk of respiratory, digestive, or skin infections.
Conclusion
The horse’s immune system relies on a complex structure combining innate and adaptive defences.
Its effectiveness depends on multiple elements: quality of colostrum, living conditions, diet, workload, and preventive healthcare.
Understanding how it functions provides a more global view of equine health and highlights the importance of vaccination, prevention, and an appropriate environment.
