Unless antigen is rapidly phagocytosed and entirely degraded (an uncommon event), the acquired immune response is recruited via recognition of antigen by the highly specific receptors on the surface of B and T cells. This response begins in The spleen for circulating antigen Regional lymph nodes for tissue antigen Mucosa-associated lymphoid tissues (eg, tonsils, adenoids, Peyer patches) for mucosal antigen For example, Langerhans dendritic cells in the skin phagocytose antigen and migrate to local lymph nodes; there, peptides derived from the antigen are expressed on the surface of dendritic cells within class II major histocompatibility complex (MHC) molecules, which present the peptide to CD4 helper T (Th) cells.