From the moment of birth, the host is constantly exposed to a wide variety of bacteria and viruses. In general, the host manages to either eliminate or ward off these invading organisms, and a symbiosis is achieved between microbes and the host. How does this occur? There are two major pathways to achieve this resistance: nonspecific and adaptive.
Nonspecific or natural resistance refers to barriers, secretions, and normal flora that make up our external defenses. Phagocytes and complement are also involved. Mechanical barriers are highly effective, and the skin (our largest organ) is highly suited to this protection (see Figure 4.1); loss of a major part of the skin (secondary to burns, acids, etc.) immediately exposes the host to marked susceptibility to infection. The mucosal lining of mouth and respiratory tract is another excellent defense mechanism. Yet, a defect in the mucosal lining of the respiratory tract, which occurs in cystic fibrosis, results in a heightened susceptibility to many infections. These are examples of a defect in the epithelium or epithelial lining. In general, however, it is the mobilization of the phagocytic cells such as monocytes/macrophages and polymorphonuclear leukocytes that ingest invading microorganisms and kill them.
The polymorpholeukocytes are a large pool of phagocytic cells that are both circulatory and in the bone marrow. Invading organisms trigger an inflammatory cascade, which stimulates these cells to adhere to vascular epithelium and actively migrate toward the infection. Phagocytosis is promoted by opsonins (usually IgG antibody) and complement.
The macrophages reside in the sub-epithelial tissues of the skin and intestine and line the alveoli of the lungs.