Antigen presenting cells are a functionally defined group of cells which are able to take up antigens and present them to T lymphocytes in a recognizable form (in the groove of an MHC class II molecule). Although many cells can do this the cells which are most efficient, the so-called "professional antigen presenting cells", are macrophages and dendritic cells. These are professional cells because they are highly effective at producing the "second signal" required for T cell activation. The APC first internalizes the antigen (maybe in the form of a bacteria or bacterial product), processes it (breaks it down into antigenic peptides [epitopes] by digesting it with lysosyme) and then expresses the antigen fragment on its surface in the groove of an MHC class II molecule. Now the antigen is in the form recognizable by T cells. Antigen presentation by APC is a required first step in Th cell activation .