In this episode, Julia Felippe, MV, MS, PhD, Dipl. ACVIM (LAIM), discussed the equine immune system. She described its primary components, explained the different types of immunity, and discussed ways veterinarians can help support immune health in their patients.
Components of the Equine Immune System
The equine immune system is comprised of the innate immune system and the adaptive immune system. The innate immune system responds promptly to pathogens and allergens, and the adaptive immune system increases the specificity for certain trigger allergens or antigens. The adaptive immune system can also develop memory.
The innate immune system if comprised of phagocytes, which detect and remove pathogens. The adaptive immune system is comprised of lymphocytes, including B cells and T cells that carry receptors on the cell surface to detect specific sequences or processed proteins.
Humoral Immunity in Horses
Felippe explained that humoral immunity in horses is primarily provided by differentiation of B cells, which produce antibodies. B cells develop during gestation and expand with reasonable diversity in their receptors, which carry an antibody or an immunoglobulin structure with a certain specificity for their matching target.
“When the foal is born, it has what we call a pre-immune repertoire of B cells that can produce antibodies that match to a large number of targeting sequences of structures,” Felippe said. “Because the foal is naïve to those antigens at birth, those B cells need to go through cycles of differentiation to increase the population that recognizes specific antigens and pathogens and then increase the affinity of the receptors for those specific pathogens to create the best antibody for that specific pathogen.”
It takes time for humoral immunity to develop after birth, but it eventually serves several important functions, such as neutralizing pathogens to block infections, facilitating phagocytosis, activating complement to kill pathogens, and activating mast cells.
Vaccination is important for humoral immunity, as vaccines facilitate this process and allow antibodies to participate in many mechanisms of protection.
Key Components of Innate Immunity in Horses
“Innate immunity is an area of the immune system that is prepared to act right away,” Felippe said. “If there is a threat in the body, let’s say caused by pathogens, the cells of the innate immune system named neutrophils and macrophages are equipped to first recognize the presence of those pathogens and then secondly to remove them by phagocytosis and kill them intracellularly.”
Phagocytes recognize the presence of pathogens through the ligand of their toll-like receptors (TLRs). The TLRs can bind to pathogen structures, which triggers intracellular activation. Phagocytes have different types of TLRs that can recognize different pathogen-associated patterns, which increases the ability to recognize many types of pathogens and microorganisms through similar receptors.
Other components of the innate immune system include fibrocytes, which have different types of mechanisms they use for phagocytosis, and dendritic cells, which detect the presence of pathogens and bring pathogen information to the lymph nodes.
Cell-Mediated Immunity in Horses
Cell-mediated immunity comes from the activation and expansion of T cells. There are two main types of T cells: CD4+ T cells (helper T cells) and CD8+ T cells (cytotoxic or killer T cells).
In horses, TH1, TH2, and TH17 CD4+ T cells are particularly important. TH1 CD4+ T cells are characterized by their interferon gamma production and the activation of cell-mediated cytotoxicity, particularly by CD8+ T cells. They reinforce the activation and expansion of CD8+ T cells to kill cells infected with intracellular pathogens such as viruses or intracellular bacteria.
TH2 CD4+ T cells help the immune system’s humoral response and allow B cells to differentiate rapidly upon antigen encounter. They also serve as checkpoints for B cell differentiation. These cells play a major role in allergies mediated by antibodies.
TH17 CD4+ T cells promote inflammatory responses by neutrophils and play a role in many inflammatory processes, such as asthma. They are important in triggering the inflammatory response to allow arrival and activation of inflammatory cells at the site where they’re needed.
How Veterinarians Can Support Immune Health in Horses
Felippe said a good immune system relies on three aspects: balanced nutrition, control of stress, and a clean environment. She said nowadays, adequate nutrition is readily available for horses, and by meeting the horse’s minimum nutrition requirements, you will supply what the immune cells need.
Control of stress can be more challenging. “Horses become quite stressful during transportation, when they join new herds, or when they are in intensive conditioning exercise,” Felippe said. “In those conditions, it would be nice to have some kind of planning to minimize stress.” She recommended getting horses used to the trailer before going on trips, giving them rest periods during long trips, acclimatizing horses to a new herd, and allowing for resting periods in conditioning programs.
Environments that are too wet or too dry can cause different types of diseases and facilitate infectious or allergic processes. Keeping horses in well-ventilated areas and avoiding overcrowded environments can help their overall health.
About Dr. Julia Felippe
Julia Felippe, MV, MS, PhD, Dipl. ACVIM, received her veterinary degree from UNESP-Campus Botucatu, Brazil, in 1989 and pursued equine practice in her native country before an internship and residency in equine internal medicine at Kansas State University from 1994-1998. She is board certified by the American College of Veterinary Internal Medicine. She has a Master of Science degree from Kansas State University (1997) and a Doctor of Philosophy degree in immunology and immunogenetics from Cornell University (2002). She has been a faculty at Cornell University College of Veterinary Medicine since 2002 and runs a research program studying equine developmental immunology and immunodeficiencies as the Head of the Equine Immunology Laboratory. Felippe also serves as the Executive Director of the Cornell Veterinary Educators Academy and the Vice-President of the Veterinary and Comparative Clinical Immunology Society (VCCIS).
Related Reading
- Equine Allergy Testing
- Equine Insect Bite Hypersensitivity
- Effects of Transport and Exercise on Intestinal Permeability in Horses
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