How many times have you seen this something like this on an antibody product sheet: “Suggested positive control – Tonsil”? Most of us are familiar with the palatine tonsils – particularly if we have ever had tonsilitis. These masses of lymphoid tissue are in the walls of the pharynx and nasopharynx and are important to developing immunity to pathogens entering the body through the oral cavity. Tonsil structure and cellular composition important to this immune function. This, and their relative availability, also makes them a handy laboratory tool. But how do you know if your reagents are staining the right cells in the tonsil? In this session, we will review the structure of the tonsil and examples of staining from special stains and immunohistochemistry. The goal is to gain a better understanding of the components of the tonsil so you can feel more confident in your interpretation of staining results.