Answer:
The immune system has three lines of defense: 1) non-specific physical and chemical barriers, 2) non-specific immune responses, and 3) specific immune responses. The third line of defense involves highly specialized cells called lymphocytes, including B cells and T cells, which produce antibodies and directly attack infected cells. The body remembers pathogens through a process called immunological memory, maintained by memory cells that persist in the body and can quickly produce a specific immune response upon subsequent exposures to the same pathogen. Memory cells have a longer lifespan than other B and T cells, allowing for long-lasting immunity and the basis for many vaccines.