Antigen-presenting cells

Antigen-presenting cells (APCs) are primarily phagocytic cells: APCs recognize a foreign cell by the absence of HLA I ( MHC I) and phagocytose it, then expose it to the surface of its antigen embedded in the molecular pocket of individually specific HLA I and HLA II (MHC) proteins. A naive T cell or effector cell binds to APC cells.
 * dendritic cells,
 * macrophages or
 * B-lymphocytes (possibly activated T-lymphocytes).

Recognition of the HLA-peptide complex by a specific TCR receptor, costimulation (e.g., CD28), and cytokine expression (interleukin-2) is required to activate a T cell or effector cell.

In the case of a naive T-cell, activation triggers clonal expansion and differentiation (T-helper, T-cytotoxic, ...), in the case of an effector T-cell, it activates effector functions.