Chemical Barriers
Certain chemicals also contribute to resistance of the skin and mucous membranes.
Sebaceous (oil) glands secrete Sebum which forms a protective film over the surface of the skin. Sebum inhibits the growth of certain pathogenic bacteria and fungi. Tears also contain Lysozyme , an enzyme capable of breaking down cell walls of bacteria. Lysozyme is also present in saliva, perspiration, nasal secretions, and tissue fluids.
Gastric Juices destroys microbes in the stomach due to the low pH.
Perspiration helps flush microbes from the surface of the skin.
Vaginal & Seminal Secretions move microbes out of the body.
Second Line of Defense: Internal Defenses
If pathogens are able to penetrate physical and chemical barriers of the skin, they encounter a second line of defense : internal antimicrobial substances, phagocytes, natural killer cells, inflammation and fever.
A) Antimicrobial Substances
There are 4 main types :
1) Interferons : protect uninfected host cells from viral infection.
Lymphocytes, macrophages and fibroblasts infected with viruses produce proteins called Interferons (IFNs). IFNs are released from infected cells and diffuse to uninfected neighboring cells where they induce synthesis of antiviral proteins that interfere with viral replication. They do not prevent viruses from attaching to and penetrating host cells but rather prevent replication.
2) Complement System: enhance the body`s ability to fight infection
A group of normally inactivated proteins in blood plasma and on plasma membranes make up the Complement System. As activated, they “complement” or “enhance” certain immune functions such as cytolysis (bursting of microbes), promotes phagocytosis, and contributes to inflammation.
3) Iron-Binding Proteins: decrease iron in body to decrease bacterial replication
These inhibit the growth of certain bacteria by reducing the amount of available iron. Examples are found in the blood and tissues, milk, saliva, mucus, liver, spleen, and red bone marrow.
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