Immunity is the physiological state in which an organism resists or overcomes infection by a specific pathogen. In horses it is divided into two broad systems that operate in parallel. Innate immunity is the rapid, non-specific first line of defense: physical barriers such as intact skin and mucous membranes, phagocytic cells (neutrophils and macrophages) that engulf and destroy invaders, and soluble proteins of the complement system that tag pathogens for destruction or directly lyse bacterial membranes. Innate responses activate within minutes to hours and do not require prior exposure to the pathogen.
Adaptive immunity develops over days to weeks after exposure and is distinguished by specificity and memory. B lymphocytes produce antibodies that bind to a particular pathogen's antigens, neutralizing it or marking it for phagocytosis. T lymphocytes coordinate the cellular response: helper T cells amplify the antibody response and cytotoxic T cells directly kill virus-infected host cells. After the pathogen is cleared, long-lived memory cells persist in the circulation; re-exposure to the same antigen triggers a faster and larger secondary response that typically eliminates the pathogen before clinical disease develops. This immunological memory is the biological basis on which priming the memory pool through vaccination programs are designed—by presenting antigens without live disease, vaccines prime the memory pool.
Passive immunity is the temporary transfer of pre-formed antibodies from one individual to another. In horses the most critical instance is the transfer of immunoglobulins from the mare's colostrum to the neonatal foal dependent on colostral transfer in the first twelve to eighteen hours after birth. The equine placenta does not allow immunoglobulin passage in utero, so the newborn foal is entirely dependent on colostral antibody absorption through the intestinal wall during this narrow window; failure of passive transfer (FPT)—defined as serum IgG below 400 mg/dL at twenty-four hours of age—leaves the foal highly susceptible to septicemia, pneumonia, and joint infections. Testing IgG at eighteen to twenty-four hours is standard neonatal care, and plasma transfusion from a healthy donor corrects FPT when detected early. The mare should receive her booster vaccinations four to six weeks before foaling to maximize the immunoglobulin concentration of the colostrum she produces.
Further Reading
- immunity (Wikipedia)
- For a clinical overview of equine immune function and disorders: the immune system of horses — MSD Veterinary Manual