The adaptive or acquired immune system is a second line of defense against pathogens. This defense develops over a longer period of time, taking several days to even weeks to develop. Compared to the innate response, adaptive immunity is much more complex because it involves antigen-specific responses and immunologic "memory" of pathogenic organisms. Exposure to a specific antigen on an invading pathogen stimulates production of immune cells that target the pathogen for destruction; subsequent responses to the same pathogen are stronger and more efficient because the antigen is "remembered." The major mediators of the adaptive immune response are specialized white blood cells, namely B lymphocytes (B cells) and T lymphocytes (T cells). B cells produce antibodies and are responsible for humoral immunity, while T cells are responsible for cell-mediated immunity. Although the innate and adaptive immune systems have unique functions, their components interact and work together to protect the body from infection and disease.