Question: where are antigen a and antigen b located

title: Antigen-presenting cell
text: T cells must be activated by interacting with a professional APC presenting an antigen which their T cell receptor recognizes before they can divide and perform their function . The APC involved in activating T cells is usually a dendritic cell . T cells can not recognize and therefore can not respond to , ' free ' or soluble antigens . They can only recognize and respond to antigen that has been processed and presented by cells via carrier molecules like MHC molecules . Helper T cells can recognize exogenous antigen presented on MHC class II ; cytotoxic T cells can recognize endogenous antigen presented on MHC class I. Most cells in the body can present antigen to CD8+ cytotoxic T cells via MHC class I ; however , the term `` antigen - presenting cell '' is often used specifically to describe professional APCs . Such cells express MHC class I and MHC class II molecules and can stimulate CD4+ helper T cells as well as cytotoxic T cells .