So let's begin with the helper T cells. So helper T cells basically release special types of chemicals into the surrounding blood or the surrounding lymph and they help other cells, other immune cells, actually mature into their specialized form and we'll talk more about that in the next several lectures. We also have killer T cells, not to be confused with natural killer cells. These two cells are two different types of cells. So killoty cells are responsible for using their protein, their membrane bound protein found on the cell membrane to basically seek out specific types of pathogens and kill off those pathogens by releasing a special type of chemical into those pathogen cells. Now, what about memory T cells?