Tonsils

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The tonsils are a collection of lymphoid tissue found in the mucosa of the pharynx which form an important part of our immune system and act as the first line of defense against ingested or inhaled pathogens.
There are four primary groups of tonsils (pharyngeal, tubal, palatine and lingual),which are arranged into a ring around the naso- and oropharynx and are collectively known as the pharyngeal lymphoid ring (of Waldeyer).

The tonsils are all classified as mucosa-associated-lymphoid-tissue (MALT).

• Due to their proximity to the oral cavity, the palatine and lingual tonsils are covered by nonkeratinized squamous epithelium. A variable number of tonsillar pits interrupt the surface; they open into deep invaginations known as tonsillar crypts, which can accumulate detritus and microorganisms. In the case of the lingual tonsils, the excretory ducts of lingual glands also open into these crypts.
• Tubal and pharyngeal tonsils, on the other hand, are covered by pseudostratified ciliated columnar epithelium (a.k.a. respiratory epithelium) containing goblet cells. The pharyngeal tonsil does not contain crypts in the same manner as those seen in palatine/lingual tonsils. Instead, its surface presents longitudinal folds between which are clefts that receive the excretory ducts of seromucous/mixed glands located beneath the lymphoid tissue. The tubal tonsils have a very similar structure.

Deep to the epithelial surfaces of all tonsils are numerous lymphoid nodules (B-cell regions/cortex) which are separated by paracortex (T-cell regions). 

Take a detailed look at the palatine and pharyngeal tonsils in the galleries below.

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