Physical and Chemical Barriers
Innate Defense
Mechanisms
Physical and Chemical Barriers
The unbroken surface of most animals provides a barrier to invading organisms. It may be tough and cornified, as in many terrestrial vertebrates, or sclerotized, as in arthropods. Soft outer surfaces are usually protected by a layer of mucus, which lubricates the surface and helps dislodge particles from it.
A variety of antimicrobial substances are present in body secretions of vertebrates. Chemical defenses present in many vertebrates include a low pH in the stomach and vagina and hydrolytic enzymes in secretions of the alimentary tract. Mucus is produced by mucous membranes lining the digestive and respiratory tract of vertebrates and contains parasiticidal substances such as IgA and Lysozyme. IgA is a class of antibody that can cross cellular barriers easily and is an important protective agent in the mucus of the intestinal epithelium. IgA is present also in saliva and sweat. Lysozyme is an enzyme that attacks the cell wall of many bacteria.
Various cells, including those involved in the acquired immune response, liberate protective compounds. A family of low-molecular-weight glycoproteins, the interferons, are released by a variety of eukaryotic cells in response to invasion by intracellular parasites (including viruses) and other stimuli. Tumor necrosis factor (TNF) is produced mainly by cells called macrophages (see following text). TNF is a major mediator of inflammation and in sufficient concentration causes fever. Fever in mammals is one of the most common symptoms of infection. The protective role of fever, if any, remains unclear, but high body temperature may destabilize certain viruses and bacteria.
The intestine of most animals harbors a population of bacteria that seem not to be harmed by host defenses, nor do they elicit any protective defense response. In fact, the normal intestinal microflora in vertebrates tends to inhibit establishment of pathogenic microbes.
Substances in normal human milk can kill intestinal protozoa such as Giardia lamblia and Entamoeba histolytica , and these substances may be important in protection of infants against these and other infections. Antimicrobial elements in human breast milk include lysozyme, IgA, interferons, and leukocytes (white blood cells,). Some species of mammals are susceptible to infection with parasites such as Schistosoma mansoni, and others are partially or completely resistant. Without mediation by antibody, macrophages of more resistant species (rats, guinea pigs, rabbits) kill schistosome juveniles, but macrophages of susceptible species do not.
Complement is a series of enzymes that are activated in sequence as a host response to invading organisms. Activation of complement by the classical pathway (so called only because it was discovered first) depends on fixed antibody and so is an effector mechanism in the acquired immune response following text. Complement activated by the alternative pathway, is an important innate defense against invasion by bacteria and some fungi. Classical and alternative pathways share some, but not all, components. In the alternative pathway, the first component is activated spontaneously in the blood and binds to cell surfaces. This event initiates a cascade of activations, ultimately resulting in cell lysis. The host’s own cells are not lysed by the alternative pathway because regulatory proteins rapidly inactivate the first active component of complement when it binds to host cells but not to foreign cells.
Physical and Chemical Barriers
The unbroken surface of most animals provides a barrier to invading organisms. It may be tough and cornified, as in many terrestrial vertebrates, or sclerotized, as in arthropods. Soft outer surfaces are usually protected by a layer of mucus, which lubricates the surface and helps dislodge particles from it.
A variety of antimicrobial substances are present in body secretions of vertebrates. Chemical defenses present in many vertebrates include a low pH in the stomach and vagina and hydrolytic enzymes in secretions of the alimentary tract. Mucus is produced by mucous membranes lining the digestive and respiratory tract of vertebrates and contains parasiticidal substances such as IgA and Lysozyme. IgA is a class of antibody that can cross cellular barriers easily and is an important protective agent in the mucus of the intestinal epithelium. IgA is present also in saliva and sweat. Lysozyme is an enzyme that attacks the cell wall of many bacteria.
Various cells, including those involved in the acquired immune response, liberate protective compounds. A family of low-molecular-weight glycoproteins, the interferons, are released by a variety of eukaryotic cells in response to invasion by intracellular parasites (including viruses) and other stimuli. Tumor necrosis factor (TNF) is produced mainly by cells called macrophages (see following text). TNF is a major mediator of inflammation and in sufficient concentration causes fever. Fever in mammals is one of the most common symptoms of infection. The protective role of fever, if any, remains unclear, but high body temperature may destabilize certain viruses and bacteria.
The intestine of most animals harbors a population of bacteria that seem not to be harmed by host defenses, nor do they elicit any protective defense response. In fact, the normal intestinal microflora in vertebrates tends to inhibit establishment of pathogenic microbes.
Substances in normal human milk can kill intestinal protozoa such as Giardia lamblia and Entamoeba histolytica , and these substances may be important in protection of infants against these and other infections. Antimicrobial elements in human breast milk include lysozyme, IgA, interferons, and leukocytes (white blood cells,). Some species of mammals are susceptible to infection with parasites such as Schistosoma mansoni, and others are partially or completely resistant. Without mediation by antibody, macrophages of more resistant species (rats, guinea pigs, rabbits) kill schistosome juveniles, but macrophages of susceptible species do not.
Complement is a series of enzymes that are activated in sequence as a host response to invading organisms. Activation of complement by the classical pathway (so called only because it was discovered first) depends on fixed antibody and so is an effector mechanism in the acquired immune response following text. Complement activated by the alternative pathway, is an important innate defense against invasion by bacteria and some fungi. Classical and alternative pathways share some, but not all, components. In the alternative pathway, the first component is activated spontaneously in the blood and binds to cell surfaces. This event initiates a cascade of activations, ultimately resulting in cell lysis. The host’s own cells are not lysed by the alternative pathway because regulatory proteins rapidly inactivate the first active component of complement when it binds to host cells but not to foreign cells.
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