Answer the following question given this paragraph: 

After an infection has been brought under control, most cytotoxic T cells die off. However, a few remain as memory cells. If the same pathogen enters the body again, the memory cells mount a rapid immune response. They quickly produce many copies of cytotoxic T cells specific to the antigen of that pathogen.


Q: After an infection has been brought under control and most cytotoxic t cells die off, a few remain as what?


A:
memory cells