Final answer:
B and T cells are part of the immune system, with naïve cells being inactive until they encounter pathogen-specific antigens, at which point they become activated. Activated B cells create antibodies, while T cells mediate cellular responses to fight infection, and memory cells formed post-infection confer lasting immunity.
Step-by-step explanation:
The student's question concerns why we have B and T cells in our body for pathogens that have not yet been encountered. These cells are part of the adaptive immune system and are fundamental in fighting specific pathogens when they invade the body. Our bodies contain naïve B and T cells, which are lymphocytes that have not yet been exposed to pathogenic antigens. Their role is to recognize and respond to antigens, which are proteins found on the surfaces of pathogens such as bacteria and viruses. When naïve B and T cells encounter these foreign proteins, they become activated.
Once activated, B cells produce antibodies, which specifically bind to the pathogen's antigens and mark them for destruction by phagocytes. T cells, on the other hand, stimulate the cell-mediated immune response which battles cells infected by pathogens. After an infection, most activated B and T cells die, but some become memory cells. These memory cells grant the body immunity, allowing for a faster and more potent response if the pathogen ever invades again.