Cell-mediated immunity involves the action of T cells. CD8+ cytolytic T cells can kill target cells directly. CD4+ helper cells can activate macrophages to become more effective at killing the organisms they ingest. This process is also considered to be cell-mediated immunity, although, again, a cell of the innate system carries out the ultimate effector function. Cells of the innate system are also needed to initiate humoral and cell-mediated responses. Dendritic cells and macrophages ingest organisms, digest them into peptides, and present them to T cells and B cells in a way that causes antigen-specific lymphocytes to proliferate and differentiate into effector cells.
Reference: Paul WE (ed): Fundamental Immunology, 5th ed. Philadelphia, Lippincott Williams & Wilkins, 2003.