Heterogeneous group of immunocompetent cells that mediates the cellular immune response by processing and presenting antigens to the T-cell receptor. Traditional antigen-presenting cells include MACROPHAGES; DENDRITIC CELLS; LANGERHANS CELLS; and B-LYMPHOCYTES. Follicular dendritic cells (DENDRITIC CELLS, FOLLICULAR) are also considered to be antigen-presenting cells by some authors.