Nonspecific defenses include chemical and cellular processes

Pathogens that penetrate the body's outer and inner surfaces encounter more complex nonspecific defenses that involve the secretion of various defensive proteins as well phagocytic cells.

complement proteins. Vertebrate blood contains about 20 different antimicrobial proteins that make up the complement system. These proteins, in different combinations, provide three types of defenses. In each type, the complement proteins act in a characteristic sequence, or cascade, with each protein activating the next:

► They attach to microbes, which helps phagocytes recognize and destroy them.

► They activate the inflammation response and attract phagocytes to site of infection.

► They lyse (burst) invading cells such as bacteria.

interferons. When cells are infected by a virus, they produce small amounts of antimicrobial proteins called interferons that increase the resistance of neighboring cells to infection by the same or other viruses. Interferons have been found in many vertebrates and are one of the body's first lines of nonspecific defense against the internal spread of viral infection.

Interferons differ from species to species, and each vertebrate species produces at least three different interferons. All interferons are glycoproteins (proteins with attached carbohydrate groups) consisting of about 160 amino acids. By binding to receptors in the plasma membranes of uninfected cells, interferons stimulate a signaling pathway that results in the inhibition of viral reproduction inside the infected cells.

PHAGOCYTES AND RELATED CELLULAR DEFENSES. Phagocytes provide another important nonspecific defense against pathogens that penetrate the surface of the host. Some phagocytes travel freely in the circulatory system; others can move out of blood vessels and adhere to certain tissues. Entire pathogenic cells, entire viruses, or fragments of these invaders can become attached to the membrane of a phagocyte (Figure 18.3), which ingests them by phagocytosis. When lysosomes fuse with the phagosome, the pathogens are degraded by lysosomal enzymes (see Figure 4.13b).

Several types of phagocytes play roles in nonspecific defenses (see Figure 18.2):

► Neutrophils are the most abundant phagocytes, but they are relatively short-lived. They recognize and attack pathogens in infected tissue.

► Monocytes mature into macrophages, which live longer than neutrophils and can consume large numbers of pathogens. Some macrophages roam through the body; others reside permanently in lymph nodes, the spleen, and certain other lymphoid tissues, "inspecting" the lymph for pathogens.

► Eosinophils are weakly phagocytic. Their primary function is to kill parasites, such as worms, that have been coated with antibodies.

► Dendritic cells have highly folded plasma membranes that can capture invading pathogens.

Phagocytes Legionella

18.3 A Phagocyte and Its Bacterial Prey Some bacteria (which appear yellow in this artificially colored scanning electron micrograph) have become attached to the surface of a phagocyte in the human bloodstream. Many of these bacteria will be engulfed by the phagocyte and destroyed before they can multiply and damage the human host. A single phagocyte can digest many bacteria.

18.3 A Phagocyte and Its Bacterial Prey Some bacteria (which appear yellow in this artificially colored scanning electron micrograph) have become attached to the surface of a phagocyte in the human bloodstream. Many of these bacteria will be engulfed by the phagocyte and destroyed before they can multiply and damage the human host. A single phagocyte can digest many bacteria.

nonphagocytic cells. A class of nonphagocytic white blood cells, known as natural killer cells, can distinguish virus-infected cells and some tumor cells from their normal counterparts and initiate the lysis of these target cells. In addition to this nonspecific action, natural killer cells form part of the specific defenses, as we will describe later in this chapter.

INFLAMMATION. The body employs the inflammation response in dealing with infection or with any other process that causes tissue injury, either on the surface of the body or internally. The damaged body cells cause the inflammation by releasing various substances. Cells adhering to the skin and linings of organs, called mast cells, release a chemical signal, called histamine, when they are damaged, as do white blood cells called basophils.

You have no doubt experienced the symptoms of inflammation: redness and swelling, accompanied by heat and pain. The redness and heat of inflammation result from his-tamine-induced dilation of blood vessels in the infected or injured area (Figure 18.4). Histamine also causes the capillaries (the smallest blood vessels) to become leaky, allowing blood plasma and phagocytes to escape into the tissue, causing the characteristic swelling. The pain of inflammation results from increased pressure (from the swelling) and from the action of leaked enzymes.

In damaged or infected tissue, complement proteins and other chemical signals attract phagocytes—neutrophils first, and then monocytes, which become macrophages. The macrophages, which engulf the invaders and debris, are responsible for most of the healing associated with inflammation. They produce several cytokines, which, among other functions, signal the brain to produce a fever. This rise in

18.4 Interactions of Cells and Chemical Signals in Inflammation The histamine-induced swelling of the inflammation reaction is accompanied by redness, heat, and pain.The chemical signals associated with the inflammation reaction attract the phagocytes that clear up the pathogens and damaged cells.

Skin And Body Membrane Defenses

3) Histamine causes the capillaries to dilate and become leaky; complement proteins attract phagocytes.

body temperature inhibits the growth of the invading pathogen. Cytokines may also attract phagocytic cells to the site of injury and initiate a specific response to the pathogen.

Following inflammation, pus may accumulate. It is composed of dead cells (neutrophils and the damaged body cells) and leaked fluid. A normal result of inflammation, pus is gradually consumed and digested by macrophages.

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Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • sara
    Where are the cells that have phagocytic properties skin?
    8 years ago
  • anne
    How is a pathogen recognized in the body?
    8 years ago
  • veli-pekka
    Are neutrophils a nonspecific defenses?
    8 years ago
  • Ruth
    What destroys dead or damaged body cells?
    8 years ago
  • SARAH
    What enzymes are important in nonspecific defense?
    8 years ago
  • geronimo
    How pathogens overcome phagocytosis?
    8 years ago
  • Giacinto
    Are complement proteins part of the specific and nonspecific defenses ?
    8 years ago
  • LUISELLA
    How do phagocytes provide nonspecific defense?
    8 years ago

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