Final answer:
A lack of memory cells affects the primary immune response.
Step-by-step explanation:
The statement is True. A lack of memory cells mainly affects the primary immune response. Memory cells are antigen-specific B or T lymphocytes that do not differentiate into effector cells during the primary immune response. However, upon re-exposure to the same pathogen, they can immediately become effector cells and mount a rapid and robust secondary immune response. Memory cells are crucial for long-term immunity to a pathogen.