professional antigen presenting cell A cell capable of processing and presenting lipid and protein antigens to T cells in order to initiate an immune response. APC CL:0000145 Note change of name; nearly all somatic cells can present antigens to T cells via MHC Class I complexes leading to effector responses, but professional antigen presenting cells constituitively express MHC Class II as well as constimulatory molecules, and thus can initiate immune responses via T cells. TAO:0009088 teleost_anatomy leukocyte