c. Direct Personal Contact. Infections may be transmitted via direct personal
contact. Examples include hepatitis and syphilis.
d. Opportunistic Microorganisms. Normal flora microbes may become
opportunistic pathogens under certain circumstances. Examples include the bacteria
causing dental caries and periodontal disease.
DEFENSE MECHANISMS OF THE BODY
The body has developed defense mechanisms to control and to cope with the
constant attack of microorganisms.
a. The Skin and the Respiratory Tract. The skin acts as the first line of
defense by being bactericidal and fungicidal through sebaceous secretion of fatty acids
and by the presence of native bacteria. The respiratory tract acts as an efficient air
filter. The tortuous passage of air through the nasal turbinate effectively filters objects
heavier than air (for example, dust). Cilia in the bronchi act in a flagellating (sweeping)
manner to expel foreign material and to bring it to the throat where it is swallowed or
b. White Blood Cells. Another defense mechanism is the phagocytic action of
white blood cells and macrophages in the reticuloendothelial system (RES). While
white blood cells circulate throughout the body, they concentrate at sites of serious
infection and inflammation. Fixed phagocytic cells (which line vascular and lymph
channels), as well as wandering phagocytic cells, ingest invading agents in tissues.
c. The Immune System. The immune system is a complex phenomenon
involving antigens and antibodies. Antigens are foreign proteins that elicit immune
reactions in the body. Antibodies are substances manufactured by the body in
response to an antigen. The antibody combines with the antigen to remove or
deactivate it. Active and passive are two types of immunities. Active immunity is
achieved when a person manufactures his own antibodies in response to an antigen.
Passive immunity is achieved when preformed antibodies are administered. These
preformed antibodies are manufactured by another person or an animal that was
previously actively immunized against a specific antigen.
d. The Alimentary Canal. The alimentary canal is a defense mechanism that
destroys harmful bacteria in the stomach by gastric acids (pH 1.0 to 3.5). In addition,
the abundant population of microorganisms existing in the lower colon produces
essential vitamins (such as vitamin K, biotin, and pyrodoxine), indirectly supplementing
other defense mechanisms.
e. The Eye. The mucous membrane of the eye is still another defense
mechanism. It resists infection by lacrimal secretions (tearing) containing wandering
phagocytes and lysozyme (an enzyme which destroys bacteria).