Figure 3. Routes of immunization. Different vaccination routes elicit different mucosal IgA responses. In general, the �common mucosal immune� system is more restricted than previously thought�the strongest response takes place at the vaccine-exposed mucosa and the second best response at adjacent mucosae. Oral immunization leads to an immune response in parts of the gut, as well as mammary and salivary glands; rectal immunization induces immune responses in the rectum; and vaginal immunization induces a vaginal immune response. A notable exception is nasal immunization, which not only stimulates an immune response in saliva, nasal secretions, and in the respiratory tract, but can also elicit a strong vaginal mucosal immune response. In the diagram, the red shading indicates the strength of the response.