In addition, the site of infection affects the nature of the protective immune response and may constrain research on vaccine development. For instance, many gastrointestinal parasites are not invasive and dwell only in the gastrointestinal tract, the interface with the host being the epithelial lining of the gut lumen. Since little is known about the immune effector mechanisms that function in immune hosts, there are few immunological tools to aid in selecting potential vaccine antigens. Consequently, research is guided by general biological criteria (e.g. mucosal antigen delivery) and has been mainly empirical. More basic research in mucosal immunology is required.