This chapter discusses innate immunity, which is available to us all the time, although its efficacy improves when a pathogenic organism enters the body. A very sensible, desirable and effective way of protecting the body from pathogens would be to prevent them from entering the body by erecting barriers. The best way to have need-based defence mechanisms is to have them activated by the invader. A large variety of pathogenic organisms and inanimate particulate matter are removed from the body by phagocytosis. Neutrophils are circulating phagocytes, and are most effective against pus forming bacteria. Macrophages are tissue phagocytes, and are derived from circulating monocytes. Eliminating a microorganism by phagocytosis requires that phagocytes should be available at the right place at the right time, and that the organism should adhere to the phagocyte. Interferons are released by virally infected cells. Eosinophils are specially equipped to deal with large parasites such as helminths.