In common with other animals, birds have evolved an immune system that is capable of responding to and protecting against pathogens such as Escherichia coli, Salmonella sp. Pasteurella multocida, coronavirus, and avian Influenza A virus. The immune system is generally divided into two main branches: the adaptive (specific) and the innate (non-specific) immune responses. Components of the immune system (i.e. adaptive and innate) include T-lymphocytes, B-lymphocytes, macrophages, natural killer (NK) cells, heterophils, basophils, diverse humoral communication factors such as eicosanoids and cytokines (e.g., interleukins, interferons, tumor necrosis factor) and effector molecules (e.g. immunoglobulins, complement, lysozymes, nitric oxide). Additionally, some tissues in the body are dedicated to support the immune system such as dendritic, reticular and stromal cells. Regulation, interaction, and communication between various elements of the specific and the innate immune responses result into birds possessing a highly sophisticated immune system, capable of protecting against invading pathogens.