An organism's first line of defense against attack by an invader such as a virus or bacterium is usually

A nonspecific immune response is the body's immediate reaction to foreign invaders, such as viruses or bacteria, without needing prior exposure to the specific pathogen. This response is the organism's first line of defense against such invaders. It includes physical barriers like the skin and mucous membranes, as well as various cellular and chemical mechanisms such as inflammation, fever, and the activity of phagocytes (white blood cells that engulf and destroy pathogens). These nonspecific mechanisms provide a generalized defense against a wide range of pathogens, helping to prevent infection or limit its spread until a more specific immune response can be mounted. Therefore, a nonspecific immune response serves as the organism's initial defense against attack by invaders.