cells mediating antibody-dependent cell-mediated cytotoxicity ; they are small lymphocytes without T or B cell surface markers and recognize immunoglobulin G (IgG) antibody coating the target cell by means of Fc receptors . Breakdown of the target cell is extracellular, requires direct cell-to-cell contact, and does not involve complement . Called also killer cells . a type of cell found mainly in the mucous membranes of the duodenum and jejunum; they synthesize gastric inhibitory polypeptide .