Final answer:
T cells are the primary cells that provide cellular-level immunity, making up about 80 percent of circulating lymphocytes, and are responsible for the cell-mediated immune response.
Step-by-step explanation:
The primary cells that provide cellular-level immunity are T lymphocytes (T cells), which comprise about 80 percent of circulating lymphocytes. T cells play a crucial role in the adaptive immune response, as they are directly involved in the cell-mediated immune system. This system is responsible for physically attacking foreign or diseased cells, helping to solve completely the presence of pathogens or infected cells in the body. T cells destroy these infected cells directly or release chemicals that regulate immune responses. Meanwhile, the remaining 20 percent of lymphocytes consist of B cells, which form part of the humoral immune response by producing antibodies.