Nonspecific Defense Mechanisms

Table 1.6 lists the most important mechanisms.

■ Primary defenses. The main factors in the first line of defense against infection are mechanical, accompanied by some humoral and cellular factors. These defenses represent an attempt on the part of the host organism to prevent microorganisms from colonizing its skin and mucosa and thus stave off a generalized invasion.

■ Secondary defenses. The second line of defense consists of humoral and cellular factors in the blood and tissues, the most important of which are the professional phagocytes.

■ Phagocytosis. "Professional" phagocytosis is realized by polymorphonuclear, neutrophilic, eosinophilic granulocytes—also known as microphages— and by mononuclear phagocytes (macrophages). The latter also play an important role in antigen presentation (see p. 62). The total microphage cell count in an adult is approximately 2.5 x 1012. Only 5% of these cells are located in the blood. They are characterized by a half-life of only a few hours. Microphages contain both primary granules, which are lysosomes containing lysosomal enzymes and cationic peptides, and secondary granules. Both mi-crophages and macrophages are capable of ameboid motility and chemotac-tic migration, i.e., directed movement along a concentration gradient toward a source of chemotactic substances, in most cases the complement components C3a and C5a. Other potentially chemotactic substances include secretory products of lymphocytes, products of infected and damaged cells or the N-formyl peptides (fMet-Phe and fMet-Leu-Phe).

Table 1.6 The Most Important Mechanisms in Nonspecific Defenses Against 1 Infection a Mechanical factors

Anatomical structure of skin and mucosa Mucus secretion and mucus flow from mucosa

Mucociliary movement of the ciliated epithelium in the lower respiratory tract

Digestive tract peristalsis

Urine flow in the urogenital tract b Humoral factors

Microbicidal effect of the dermal acidic mantle, lactic acid from sweat glands, hydrochloric acid in the stomach, and the unsaturated fatty acids secreted by the sebaceous glands

Lysozyme in saliva and tear fluid: splitting of bacterial murein Complement (alternative activation pathway)

Serum proteins known as acute phase reactants, for example C-reactive protein, haptoglobin, serum amyloid A, fibrinogen, and transferrin (iron-binding protein)

Fibronectin (a nonspecific opsonin); antiviral interferon

Mannose-binding protein: binds to mannose on the outer bacterial surface, thus altering the configuration and triggering alternative activation of complement c Cellular factors

Normal flora of skin and mucosa

Natural killer cells (large, granulated lymphocytes; null cells)

Professional phagocytes: microphages (neutrophilic and eosinophilic granulocytes); mononuclear phagocytes (macrophages, monocytes, etc.)

Phagocytes are capable of ingestion of both particulate matter (phagocytosis) and solute matter (pinocytosis). Receptors on the phagocyte membrane initiate contact (Fig. 1.6). Particles adhering to the membrane are engulfed, ingested and deposited in a membrane-bound vacuole, the so-called phago-some, which then fuses with lysosomes to form the phagolysosome. The bacteria are killed by a combination of lysosomal factors:

— Mechanisms that require no oxygen. Low pH; acid hydrolases, lysozyme; proteases; defensins (small cationic peptides).

— Phagocytosis of Bacteria

Lysosome Attachment ?

Bacteria + serum factors (antibodies, C3b, fibronectin)


Bacteria + serum factors (antibodies, C3b, fibronectin)


= Nonspecific



= Fc receptor

= C3b receptors

CR1 and CR3

Lysosome Attachment ?


Killing and digestion


Fig. 1.6 Encapsulated bacteria can only be effectively phagocytosed if IgG-class antibodies (Fc ligand) or the complement component C3b, or both, are located on their surfaces. The Fc and C3b ligands bind to their specific receptors on the phagocyte surface.

Killing and digestion

Fig. 1.6 Encapsulated bacteria can only be effectively phagocytosed if IgG-class antibodies (Fc ligand) or the complement component C3b, or both, are located on their surfaces. The Fc and C3b ligands bind to their specific receptors on the phagocyte surface.

— Mechanisms that require oxygen. Halogenation of essential bacterial components by the myeloperoxidase-H2O2-halide system; production of highly reactive O2 radicals (oxidative burst) such as superoxide anion (O2-), hydroxyl radical (*OH), and singlet oxygen (:O2).

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  • adolfa
    What organisms colonizes the skin or mucosal membrane?
    8 years ago
    What are some nonspecific defense defense mechanisms?
    8 years ago
  • daniel
    What are the secondary defense mechanisms in medical?
    8 years ago
  • sofia omar
    Is phagocytosis a nonspecific defense mechanism?
    6 years ago

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