Among the following, which cell type is responsible for forming memory cells?

B lymphocytes are responsible for forming memory cells. When B lymphocytes are activated by an antigen, they differentiate into plasma cells that produce antibodies. Some of these activated B cells become memory B cells, which are long-lived and can quickly respond to subsequent exposures to the same antigen. This ability to "remember" the specific antigens allows the immune system to mount a faster and more effective response upon re-exposure, which is crucial for long-term immunity.

The other cell types listed do play important roles in the immune system but are not primarily responsible for the formation of memory cells. T lymphocytes have their own subset that forms memory T cells, which are important for cell-mediated immunity but are distinct from memory B cells. Natural killer cells are involved in the innate immune response and target infected or cancerous cells but do not form memory cells. Dendritic cells serve as antigen-presenting cells that activate T and B lymphocytes, but they do not become memory cells themselves.