Tonsils are a type of mucosa-associated lymphoid tissue or MALT. MALT is a secondary lymphoid structure where the majority of lymphocyte activation occurs in the body. The tonsils are epithelia-lined collections of lymphoid nodules located in the oral and nasal cavities that provide immune surveillance of antigens (e.g. foreign substances and microorganisms) entering the body through the mouth and nose.

• Examine this section of palatine tonsil. Identify tonsillar crypts and the stratified squamous epithelium and dense connective tissue covering the large lymphoid follicles. The large, pale cells scattered among the lymphocytes in the follicles are macrophages, acting as antigen-presenting cells. As you might expect, there are also many mitotic cells within the follicles.
• Examine the images below and these slides of the pharyngeal and lingual tonsils, and compare them to the palatine tonsil. The pharyngeal tonsil is lined by respiratory epithelium, whereas the lingual tonsil is lined by stratified squamous epithelium and located near other structures within the tongue (e.g. salivary glands), which help to distinguish it from the palatine tonsils.

Clinical note: Tonsillitis, pharyngitis, and other upper respiratory tract infections are among the most common problems involving the head and neck. Tonsillectomies, which were formerly much more common than now, may be required if chronic tonsillitis causes enlargement of the affected structures to the degree that air passages are obstructed. To the right we see a pair of very large and inflamed tonsils.
 

Diffuse, mucosa-associated lymphatic tissue of other organs.