Memory B cells are B cells that although activated by the immune system, they are stored inside the circulatory system for later use, for long periods of time, possibly a whole lifetime. Like other cells such as helper T cells, killer T cells, and plasma cells they never become directly involved in the immune response to foreign objects in a living body. If a pathogen the body has already encountered invades, memory B cells can recognize the pathogen and start to divide. Quickly, they form a new generation of cells, and memory cells. The new generation kills off the pathogen so quickly the body does not become noticeably ill.