Overview of the Innate Defenses

First-line defenses are the barriers that separate and shield the interior of the body from the surrounding environment; they are the initial obstacles that microorganisms must overcome to invade the tissues. The anatomical barriers, which include the skin and mucous membranes, not only provide physical separation, but they are often bathed in secretions containing substances that have antimicrobial properties (figure 15.1). Characteristics of the components of innate immunity, including the first-line defenses, are summarized in table 15.1.

Sensor systems within the body recognize when the firstline barriers have been breached and then relay that information to other components of the host defenses. An important group

Respiratory tract

Alimentary tract

Urogenital tract

Respiratory tract

Alimentary tract

Urogenital tract

Mucous membranes

Figure 15.1 Anatomical Barriers These barriers separate the interior of the body from the surrounding environment; they are the initial obstacles microorganisms must overcome to invade tissues.The skin is shown in purple.

Mucous membranes

Figure 15.1 Anatomical Barriers These barriers separate the interior of the body from the surrounding environment; they are the initial obstacles microorganisms must overcome to invade tissues.The skin is shown in purple.

15.1 Overview of the Innate Defenses 373

Table 15.1 Summary of Important Aspects of Innate Defense

Defense Component

Protective Characteristics

First-Line Defenses

Separate and shield the interior of the body from the surrounding environment.

Physical barriers

Physically prevent microbes from accessing the tissues. Skin is a tough durable border; the outermost layers of cells constantly slough off. Mucous membranes are constantly bathed with mucus and other secretions that help wash microbes from the surfaces; some mucous membranes have mechanisms that propel microbes, directing them toward areas where they can be eliminated more easily.

Antimicrobial substances

Destroy or inhibit microbes. Lysozyme degrades peptidoglycan, peroxidase enzymes produce potent oxidizing compounds, lactoferrin sequesters iron, and defensins form pores in bacterial membranes.

Normal flora

Competitively excludes pathogens by preventing adherence, consuming nutrients, and producing toxins; the normal flora also stimulates the host defenses.

Cell Communication

Enables cells to respond to trauma or invasion in a cooperative fashion.

Surface receptors

Enable the inner workings of the cell to sense and respond to signals outside of the cell.The receptors span the cell membrane, connecting the outside of the cell with the inside. Ligands bind the receptors.


Function as chemical messengers, allowing cells to communicate. Cytokines include interleukins, colony-stimulating factors, tumor necrosis factors, chemokines, and interferons.

Adhesion molecules

Allow cells to adhere to other cells. Endothelial cells use adhesion molecules to snare passing phagocytes that are needed in tissues. Some cells use adhesion molecules to make direct contact with other cells, enabling the targeted the delivery of certain compounds to particular cells.

Sensor Systems

Detect signs of invasion or tissue damage and then destroy the invading microbes or recruit other components of the host defense.

Toll-like receptors (TLRs)

Allow a cell to "sense" the presence of microbes and respond accordingly.TLRs on the cell surface bind directly or indirectly to molecules such as peptidoglycan and lipopolysaccharide (LPS) that are uniquely associated with microbes.

Complement System

Activated forms of the complement proteins assist phagocytes in their recognition and engulfment of foreign material, assemble themselves into membrane attack complexes, and contribute to inflammation. Antibody-antigen complexes and foreign cell surfaces both trigger the activation of the complement system.


Engulf and degrade foreign material and cell debris.


Always in tissue to some extent, but more can be recruited to the site of injury. Activated macrophages have greater killing power.


Inherently have more killing power than macrophages, but a shorter life span; rapidly recruited to a site of infection.

Inflammation (a coordinated response to invasion or damage)

Contains a site of damage, localizes the response, and ultimately restores tissue function.The inflammatory process is initiated in response to microbial products, microbes, and tissue damage.This results in dilation of blood vessels, allowing fluid and cells to exit the bloodstream and enter the site of damage.


Induce cells to prepare to cease protein synthesis in the event that the cell becomes infected with a virus. Virally infected cells produce interferon, a cytokine, which diffuses to neighboring cells.


Elevates the temperature above the optimum growth temperature of most pathogens. Activates and accelerates other body defenses.

of sensors that has only recently been discovered is the toll-like receptors, which are found on the surface of a variety of different cell types. These receptors recognize families of compounds unique to microbes, enabling the cell to sense invaders and then send chemical signals to alert other components of the host's defense. Another type of sensor is a series of proteins that are always present in blood; these proteins are collectively called the complement system because they can "complement," or act in conjunction with, the adaptive immune defenses. In response to certain stimuli, the complement proteins become activated, set ting off a chain of events that results in removal and destruction of invading microbes.

Phagocytes, cells that specialize in engulfing and digesting microbes and cell debris, act as sentries, alert for signs of invasion of the body. More can be recruited from the bloodstream, serving as reinforcements at the sites in tissues where first-line defenses have been breached.

Cells of the immune system communicate with one another by producing proteins that function as chemical messengers, called cytokines. A cytokine produced by one cell

374 Chapter 15 The Innate Immune Response diffuses to another and binds to the appropriate cytokine receptor of that cell. When a cytokine binds a receptor, the receptor transmits a signal to the interior of the cell, inducing certain changes in the activities of the cell. Some types of cytokines endow cells with enhanced powers; others prompt cells to migrate to specific locations within the body.

When invading microorganisms or tissue damage is detected, inflammation ensues; this is a coordinated response involving many aspects of the innate defenses. During inflammation, the cells that line local blood vessels near the area of invasion or damage undergo changes that allow antibodies, complement proteins, and coagulation proteins in plasma, the fluid portion of the blood, to leak into tissues. Other changes allow phagocytic cells in the bloodstream to adhere to the vessels and then squeeze between cells, exiting the bloodstream. Phagocytic cells then migrate to the area of infection or damage where they ingest and destroy foreign material. Some types of phagocytes play a dual role, destroying invaders while also communicating with cells of the adaptive immune system, enlisting their far more powerful effects.

The body also has physiological defense mechanisms, such as the increase in internal body temperature called fever, which acts in several ways to discourage infection.

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