The first line of immune defense against pathogens, before adaptive immunity (antibodies, T cell responses) develops, is called innate immunity. This is a complex set of responses triggered when specific cells (macrophages, phagocytes, dendritic cells) recognize even a yet unknown pathogen by some characteristics common to most pathogens (these "signatures" are called pathogen-associated molecular patterns) through a family of pathogen recognition receptors. This activates a response that involves production of ROS, a major bactericidal mechanism, and of soluble mediators (cytokines) whose role is to amplify the host response by recruiting and activating other immune system cells.