Brucellosis is a zoonotic infectious disease of domestic animals, wild animals and humans. Innate immunity is a rapid and non-specific immune response that occurs during the early stages of Brucella invasion. Physical barriers such as epithelial cells and gastric juice secretions form the first line of defense. Humoral components such as complement and lysozyme can remove microorganisms by opsonization and bactericidal actions. Cellular components of the immune system, including macrophages, dendritic cells, neutrophils and innate T cells, have major roles in innate immunity. They recognize invading Brucella spp. by various cell surface receptors and then kill both the invading microorganisms and infected cells owing to their phagocytic or cytotoxic activity. In addition, they present Brucella antigens or produce cytokines to trigger adaptive immunity. Activated adaptive immunity consists of T helper cells, cytotoxic T cells and antigen-specific antibody-producing B cells. These can eliminate Brucella spp. effectively via antigen-specific mechanisms and by immunological memory. T cells activate bactericidal functions in macrophages by producing cytokines such as IFN-γ and by exerting cytotoxic effects on the infected cells. B cells produce antigen-specific antibodies that neutralize or opsonize the antigen. Because Brucella spp. can survive in macrophages and other host cells, Th-1 cellular immunity that enhances the bactericidal effects of phagocytic cells and the cytotoxic effects of lymphocytes is more important than humoral immunity in Brucella infection.