the difficulty of serologically demonstrating the presence of soluble antigen on the surface of apc , the observed rapid degradation of antigen by these cells , and the finding that under special circumstances peptides of a certain protein are more antigenic that the whole molecule have led to the notion that foreign antigens must be rendered immunogenic for helper t cells by internalization , processing ( probably involving enzymatic fragmentation ) , and redisplay on the membrane of apc in association with class ii mhc molecules .
C0699040
surface of apc
C0007634
cells