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.