This dissertation was motivated by the desire to further our understanding of the immune response to vaccination and with the hope of promoting the development of improved vaccine strategies. These studies have focused on the equine dendritic cell (DC) because of the important role this cell plays in initiating the immune response; DCs are unique in their ability to optimally sensitize naïve T cells and are capable of promoting the development of a range of effector T cell phenotypes. In particular, the phenotypic and functional characteristics of the equine monocytederived DC were characterized. Monocyte-derived DCs and macrophages were stimulated with UV-inactivated Escherichia coli (E. coli) and monitored for cell surface marker expression, cytokine production, and endocytic capacity. The resulting alterations in DC activation state were characterized, and the differences between DCs and macrophages were further defined. These findings contribute to our knowledge of equine DCs and demonstrate that, although non-stimulated DCs consist of a mixed population of mature and immature cells, DC maturation can be measured following induction by bacterial stimuli. A method for characterizing DC function was also developed. Relatively pure populations of equine monocyte-derived DCs were co-cultured with autologous, 5,6carboxyfluorescein diacetate succinimidyl ester (CFSE)-stained peripheral blood T cells. Multi- color flow cytometry was used to measure antigen-specific T cell proliferation, surface-marker expression, and cytokine production. The DC-induced T cell response was characterized in response to both self-antigen and vaccine antigen. These experiments confirmed the potent antigen-presenting capabilities of equine DCs, which validates their immunotherapeutic potential and supports their use as a cellular vaccine adjuvant. This system also permitted the use of DCs to study fundamental immunological processes in vitro. These findings contribute to our knowledge of the equine immune system and demonstrate the value of DCs as a research tool. A lymphoscintigraphic procedure for identifying the vaccine-draining lymph node in the horse was also established. This technique will facilitate the study of the anti-vaccine immune response and encourage the development of improved vaccination strategies, such as those using DC-based adjuvants.