Final answer:
A cell without prominent grainlike structures in its cytoplasm is an agranulocyte, with the main types being lymphocytes and monocytes involved in the immune response.
Step-by-step explanation:
A cell that does not contain prominent grainlike structures in its cytoplasm is known as an agranulocyte. These cells have a simple-shaped nucleus, sometimes indented but not distinctly lobed. The major types of agranulocytes are lymphocytes and monocytes. Lymphocytes, which are primary cells of the immune system, include B cells, T cells, and natural killer cells with various functions related to immune defense. Monocytes mature into macrophages that are phagocytic and play a role in responding to infections or injuries. Thus, the correct answer to the question is B) Agranulocyte.