The tonsils are a set of lymphoid organs consisting of large, partly encapsulated aggregations of lymphoid tissue facing into the aerodigestive tract. They are part of the body’s first line of defense and are organized in a ring surrounding the naso- and oropharynx by connective tissue. This ring is known as the Waldeyer ring and consists of one pharyngeal tonsil, two tubal tonsils, two palatine tonsils and the so-called lingual tonsils. As mentioned, the tonsils are located in close connection with the entrance to the respiratory and gastrointestinal systems because it enables the first defense against pathogens originating from the air we breathe.
The tonsils are covered by stratified squamous epithelium which forms deep irregular invaginations into the tonsils. Underlying the epithelium numerous lymphoid follicles are present. Lymphoid follicles are spherical aggregations of lymphocytes and can be either primary or secondary. Primary lymph follicles appear as homogeneous aggregations of darkly stained small lymphocytes, primarily B-cells. What separates the secondary lymph follicles from the primary is the presence of a lighter spherical area inside the aggregated darkly stained lymphocyte sphere. This area is called a germinal center and represents proliferating B-cells. Secondary lymph follicle germinal centers are surrounded by what is known as mantle zone, where the primary follicles memory and resting B-cells are found. Surrounding the mantle zone is the marginal zone which is where you find most antigen-presenting cells (APCs), such as dendritic cells and macrophages.
A typical feature of tonsils is the presence of lymphocytes that have infiltrated the squamous epithelium of tonsillar crypts and the mucosal surface.