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PRINCIPLES OF HOST DEFENSES

Host defenses are composed of two complementary, frequently interacting systems: (1) innate (nonspecific) defenses, which protect against microorganisms in general; and (2) adaptive (specific) immunity, which protects against a particular microorganism. Innate defenses can be classified into three major categories: (1) physical barriers, such as intact skin and mucous membranes; (2) phagocytic cells, such as neutrophils, macrophages, and natural killer cells; and (3) proteins, such as complement, lysozyme, and interferon. Figure 8–1 shows the role of several components of the nonspecific defenses in the early response to bacterial infection. Adaptive immunity is mediated by antibodies and T lymphocytes. Chapter 57 describes these host defenses in more detail.

FIGURE 8–1

Early host responses to bacterial infection.

There are two main types of host defenses against bacteria: the pyogenic response and the granulomatous response. Certain bacteria, such as Staphylococcus aureus and Streptococcus pyogenes, are defended against by the pyogenic (pus-producing) response, which consists of antibody, complement, and neutrophils. These pyogenic bacteria are often called extracellular pathogens because they do not invade cells. Other bacteria, such as Mycobacterium tuberculosis and Listeria monocytogenes, are defended against by the granulomatous response, which consists of macrophages and CD4-positive (helper) T cells. These bacteria are often called intracellular pathogens because they can invade and survive within cells.

INNATE (NONSPECIFIC) IMMUNITY

Skin & Mucous Membranes

Intact skin is the first line of defense against many organisms. In addition to the physical barrier presented by skin, the fatty acids secreted by sebaceous glands in the skin have antibacterial and antifungal activity. The increased fatty acid production that occurs at puberty is thought to explain the increased resistance to ringworm fungal infections, which occurs at that time. The low pH of the skin (between 3 and 5), which is due to these fatty acids, also has an antimicrobial effect. Although many organisms live on or in the skin as members of the normal flora, they are harmless as long as they do not enter the body.

A second important defense is the mucous membrane of the respiratory tract, which is lined with cilia and covered with mucus. The coordinated beating of the cilia drives the mucus up to the nose and mouth, where the trapped bacteria can be expelled. This mucociliary apparatus, the ciliary elevator, can be damaged by alcohol, cigarette smoke, and viruses; the damage predisposes the host to bacterial infections. Other protective mechanisms of the respiratory tract involve alveolar macrophages, lysozyme in tears and mucus, hairs in the nose, and the cough reflex, which prevents aspiration into the lungs.

Loss of the physical barrier provided by the skin and mucous membranes predisposes to infection. Table 8–1 describes the organisms that commonly cause infections associated with the loss of these protective barriers.

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