It is an uninterrupted circle of protective lymphoid tissue at the upper ends of the respiratory and alimentary tracts.

• Also known as Nasal Associated Lymphoid Tissue (NALT).
• Consists of the palatine, nasopharyngeal, lingual and tubal tonsils.
• Produces B and T lymphocytes.
• Involved in local defence.
• Also produce lymphocytes to send to other lymph nodes.