The first line of defense against invasion by potential pathogens and commensal microorganisms is the thin layer of epithelium that covers all these surfaces. However, the epithelium can be breached relatively easily, and so its barrier function needs to be supplemented by defenses provided by the cells and molecules of the mucosal immune system. The innate defenses of mucosal tissues, such as antimicrobial peptides and cells bearing invariant pathogen-recognition receptors, are described in Chapters 2 and 3. In this chapter we concentrate on the adaptive mucosal immune system, highlighting only those innate responses that are of particular importance to our discussion. Many of the anatomical and immunological principles underlying the mucosal immune system apply to all its constituent tissues; here we will use the intestine as our example, and the reader is referred to the general references at the end of this chapter for further details of the other sites.
12-1 The mucosal immune system protects the internal surfaces of the body.
The mucosal immune system comprises the internal body surfaces that are lined by a mucus-secreting epithelium—the gastrointestinal tract, the upper and lower respiratory tract, the urogenital tract, and the middle ear. It also includes the exocrine glands associated with these organs, such as the conjunctivae and lacrymal glands of the eye, the salivary glands, and the lactating breast (Fig. 12.1). The mucosal surfaces represent an enormous area to
The nature and structure of the mucosal immune system.
The mucosal response to infection and regulation of mucosal immune responses.
respiratory tract gastrointestinal tract urogenital tract conjunctiva esophagus stomach intestine sinus middle ear trachea lungs oral cavity lachrymal gland salivary gland Mucosal tissues of the human body mammary gland kidney uterus bladder vagina Fig. 12.1 The mucosal immune system. The tissues of the mucosal immune system are the lymphoid organs and cells associated with the intestine, respiratory tract, and urogenital tract, as well as the oral cavity, pharynx, middle ear, and the glands associated with these tissues, such as the salivary glands and lachrymal glands. The lactating breast is also part of the mucosal immune system.
be protected. The human small intestine, for instance, has a surface area of almost 400 m2, which is 200 times that of the skin.
The mucosal immune system forms the largest part of the body’s immune tissues, containing approximately three-quarters of all lymphocytes and producing the majority of immunoglobulin in healthy individuals. It is also exposed continuously to antigens and other materials entering from the environment. When compared with lymph nodes and spleen (which in this chapter we will call the systemic immune system), the mucosal immune system has many unique and unusual features (Fig. 12.2).
Fig. 12.2 Distinctive features of the mucosal immune system. The mucosal immune system is bigger, encounters a wider range of antigens, and encounters them much more frequently than the rest of the immune system—what we call in this chapter the systemic immune system. This is reflected in distinctive anatomical features, specialized mechanisms for the uptake of antigen, and unusual effector and regulatory responses that are designed to prevent unwanted immune responses to food and other innocuous antigens.
The nature and structure of the mucosal immune system.