Senior horses (≥20 years) exhibit inflamm-aging, or chronic, low-grade inflammation that occurs systemically with aging, similarly to humans. Inflamm-aging has previously been characterized in the horse in circulation as well as specifically being mediated by lymphocytes and monocytes. In humans, inflamm-aging has been associated with increased morbidity and mortality. However, in the horse, relatively little about inflamm-aging is known regarding clinical effects or factors influencing severity. The contribution of lymphocytes to inflamm-aging of senior horses was examined, specifically through determining the relationships of inflamm-aging with various other health parameters, effects of seasonality, and the extent to which inflamm-aging can be modulated by anti-inflammatory phytonutrient curcumin. The overall hypothesis of this research is that lymphocyte-mediated inflamm-aging of the senior horse is associated with various factors including season, endocrine function, body composition, and nutritional status, and may be modulated by polyphenol curcumin. The effect of season on lymphocyte-mediated inflamm-aging was examined, and senior horses exhibited elevated inflammation compared to adult horse as expected, while also exhibiting changes in inflammatory cytokine production and gene expression throughout the year. In addition to season, pituitary pars intermedia dysfunction (PPID), a common endocrinopathy in senior horses that is associated with immunosuppression, was examined in a group of senior horses to determine any effects on degree of inflamm-aging. Results indicated no significant differences between age-matched PPID and non-PPID horses for lymphocyte-mediated inflammatory cytokine production or gene expression. The immunosuppressive aspect of PPID does not appear to be associated with the degree of lymphocyte-mediated inflammation of the aged horse. Additionally, an expansive correlative study was undertaken to determine relationships between inflamm-aging and basal nutritional status, body composition, age, and PPID within a similarly-managed senior horse population. Results showed various relationships between inflammatory markers and nutritional status, particularly yielding positive associations with serum folate and with serum fatty acids C22:2n6c and C22:5n3c. Inflammation was also associated with age itself but was not associated with body composition parameters and showed mild association with PPID (and serum inflammatory C-reactive protein). As a whole, this study demonstrates that nutritional status can be associated with inflammatory markers. Similarly, many phytonutrients have exhibited anti-inflammatory properties, which may be beneficial to the senior horse exhibiting inflamm-aging. Specifically, the effects of polyphenols including curcuminoids, resveratrol, quercetin, pterostilbene, and hydroxypterostilbene on lymphocyte production of inflammatory cytokines by senior horses were examined in vitro and found to significantly reduce inflammation similarly to common non-steroidal anti-inflammatory drugs. This study led to the in vivo investigation of the effectiveness of curcumin in modulating chronic inflammation of the senior horse. No significant differences were seen between groups receiving curcumin and placebo for the various inflammatory parameters, which may be due to the dose or low bioavailability of curcumin. As a whole, this research provides further understanding of factors associated with inflamm-aging of the senior horse.