I
immunization The mechanism through which
the body protects itself from subsequent INFECTION
with the same PATHOGEN. Immunization may occur
as a natural consequence of infection or through
vaccination (also called inoculation). The IMMUNE
SYSTEMcreates unique antibodies, specialized pro-
teins that attach to B-cell lymphocytes that circu-
late in the BLOODand the LY M P H, in response to
antigens present on the cell surfaces of the
pathogen. The antibodies then respond to the
presence of the ANTIGENwithin the body, activating
a rapid IMMUNE RESPONSEto contain and neutralize
the pathogen before it establishes sufficiently to
cause illness. A VACCINEcontains a nonpathogenic
preparation of an antigen to stimulate the same
immune response without causing illness. Natural
immunization is usually lifelong. Immunization
acquired through vaccination may require a series
of vaccinations or periodic booster vaccines to
maintain adequate protection against infection.
See also ANTIBODY; ANTIBODY-MEDIATED IMMUNITY;
B-CELL LYMPHOCYTE; CHILDHOOD DISEASES; LYMPHOCYTE;
PREVENTIVE HEALTH CARE AND IMMUNIZATION.
incubation period The length of time between
the exposure to a PATHOGENand the appearance of
symptoms or illness. Incubation periods vary from
a few hours (illnesses such as CHOLERAand SHIGEL-
LOSIS) to months (illnesses such as HEPATITIS C,
TUBERCULOSIS, and AMEBIASIS). Some diseases, such
as AIDS (acquired immunodeficiency syndrome)
and HUMAN PAPILLOMAVIRUS(HPV), may have incuba-
tion periods of years. During the incubation period
the pathogen replicates until its presence reaches a
level that can overcome the immune system’s
efforts to contain it. Viral infections in particular
may be contagious during the incubation period.
See also BACTERIA; FUNGUS; HIV/AIDS; INFECTION;
PARASITE; VIRUS.infection The invasion of the body by a PATHOGEN
(microorganism capable of causing disease), some-
times called an infectious agent, that enters cells
and attempts to reproduce or replicate itself. The
pathogen may be a bacterium, FUNGUS(yeast or
mold), VIRUS, PARASITE, or prion. Not all infections
cause symptoms or illness, and some infections
may be present in the body for an extended time
before they cause disease. The length of time
between the entrance of a pathogen into the body
and the appearance of symptoms is the illness’s
INCUBATION PERIOD.
The IMMUNE SYSTEMdetects and in some way
contains most pathogens. INFLAMMATIONand FEVER,
for example, are ways in which the immune
response creates an unfavorable environment for
many pathogens. Though the common perception
that symptoms such as fever and MUSCLEaches are
the effects of the infection, they are often instead
the immune system’s methods for eliminating the
pathogen before its presence can cause illness.
Sometimes an infection is able to evade the
immune system’s efforts to eliminate it, such as
HIV (human immunodeficiency virus), the infec-
tion that ultimately results in AIDS (acquired
immunodeficiency syndrome). HIV actually
infects the cells of the immune system that would
fight its presence, restructuring their functions so
they are no longer effective.
See also ANTIBODY-MEDIATED IMMUNITY; BACTERIA;
CELL-MEDIATED IMMUNITY; IMMUNIZATION.influenza A common and potentially serious
viral INFECTION, commonly called the flu, that338