Fig. 12.3 Mucosal infections comprise one of the biggest health problems worldwide. Most of the pathogens that cause death throughout the world either are those of mucosal surfaces or enter the body through these routes. Respiratory infections are caused by numerous bacteria (such as Streptococcus pneumoniae and Haemophilus influenzae, which cause pneumonia; and Bordetella pertussis, the cause of whooping cough) and viruses (such as influenza and respiratory syncytial virus). Diarrheal diseases are caused by both bacteria (such as the cholera bacterium Cholera vibrio) and viruses (such as rotaviruses). The human immunodeficiency virus (HIV) that causes AIDS enters through the mucosa of the urogenital tract or is secreted into breast milk and passed from mother to child in this way. The bacterium Mycobacterium tuberculosis, which causes tuberculosis, also enters through the respiratory tract. Measles manifests itself as a systemic disease, but it originally enters via the oral/ respiratory route. Hepatitis B is also a sexually transmitted virus. Finally, parasitic worms inhabiting the intestine cause chronic debilitating disease and premature death. Most of these deaths, especially those from acute respiratory and diarrheal diseases, occur in children under 5 years old in the developing world, and there are still no effective vaccines against many of these pathogens. Numbers shown are the most recent estimated figures available (The Global Burden of Disease: 2004 Update. World Health Organization, 2008). *Does not include deaths from liver cancer or cirrhosis resulting from chronic infection.
PC2 (4.4%)
Fig. 12.4Composition of the commensal microbiota at (Actinobacteria), Bacteroides fragilis (Bacteroidetes), and Escherichia different mucosal surfaces in healthy humans. Panel a: coli (Proteobacteria). Panel b: principal component analysis of the the different sizes of the pie charts for different sites reflect the microbiomes isolated from the indicated human tissues, plotting number of distinct bacterial species typically present at those the first and second principal components. The primary component sites. The colon contains the greatest number of different species of microbiome variation is due to body area and accounts for (over 1000 as estimated from individual surveys). The color key 13% of the variation in microbial identity between samples taken indicates the four bacterial phyla that contain the majority of from these sites. a, adapted from Dethlefsen, L., et al.: Nature commensal species. Ubiquitous commensal bacteria include 2007, 449:811‑818. b, from Huttenhower, C., et al.: Nature 2012, Lactobacillus and Clostridium spp. (Firmicutes), Bifidobacterium spp. 486:207–214.
tract. Here, protective immunity against pathogens is essential, but many of the antigens entering these tissues are also harmless, being derived from commensal organisms, pollen, other innocuous environmental material, and, in the lower urogenital tract, seminal fluid. The fetus is a further important source of foreign antigen encountered by the normal mucosal immune system to which immune responses must be controlled.
12-2  Cells of the mucosal immune system are located both in anatomically defined compartments and scattered throughout mucosal tissues.