Bacteriologia

2 Basic Principles of Immunology
R. M. Zinkernagel
Introduction
& Resistance to disease is based on innate mechanisms and adaptive or
acquired immunity. Acquired immune mechanisms act in a specific manner
and function to supplement the important nonspecific or natural resistance
mechanisms such as physical barriers, granulocytes, macrophages, and
chemical barriers (lysozymes,etc.). The specific immune mechanisms
constitute a combination of less specific factors, including the activation of
macrophages, complement, and necrosis factors; the early recognition of
invading agents, by cells exhibiting a low level of specificity, (natural killer
cells, cd [gamma-delta] T cells); and systems geared toward highly specific
recognition (antibodies and ab [alpha-beta]T cells).
Many components of the specific immune defenses also contribute to
nonspecific or natural defenses such as natural antibodies, complement,
interleukins, interferons, macrophages, and natural killer cells. &
In the strict sense, “immunity” defines an acquired resistance to infectious
disease that is specific, i.e., resistance against a particular disease-causing
pathogen.For example, a person who has had measles once will not suffer
from measels a second time, and is thus called immune. However, such specific
or acquired immune mechanisms do not represent the only factors
which determine resistance to infection. The canine distemper virus is a close
relative of the measles virus, but never causes an infection in humans. This
kind of resistance isinnate and nonspecific. Our immune system recognizes
the pathogen as foreign based on certain surface structures, and eliminates it.
Humans are thus born with resistance against many microorganisms (innate
immunity) and can acquire resistance to others (adaptive or acquired immunity;
Fig. 2.1). Activation of the mechanisms of innate immunity, also
known as the primary immune defenses, takesplace when a pathogen
breaches the outer barriers of the body. Specific immune defense factors
are mobilized later to fortify and regulate these primary defenses. Responses
of the adaptive immune system not only engender immunity in the strict
sense, but can also contribute to pathogenic processes. The terms immunopathology,
autoimmunity, and allergy designate a group of immune
2Kayser, Medical Microbiology © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.

44 2 Basic Principles of Immunology
phenomena causing mainly pathological effects, i.e., tissue damage due to
inadequate, misguided, or excessive immune responses. However, a failed
immune response may also be caused by a number of other factors. For
instance, certain viralinfections or medications can suppress or attenuate
the immune response. This condition, known as immunosuppression, can
also result from rare genetic defects causing congenital immunodeficiency.
The inability to initiate an immune response to the body’s own self antigens
(also termed autoantigens) is known as immunological tolerance.
Anergy is the term used to describe the phenomenonin which cells involved
in immune defense are present but are not functional.
An immune response is a reaction to an immunological stimulus. The
stimulating substances are known as antigens and are usually proteins or
complex carbohydrates. The steric counterparts of the antigens are the antibodies,
i.e., immunoreceptors formed to recognize segments, roughly 8–15
amino acids long, ofthe folded antigenic protein. These freely accessible
structural elements are known as epitopes when present on the antigens,
or as antigen-binding sites (ABS) from the point of view of the immunoreceptors.
Presented alone, an epitope is not sufficient to stimulate an immunological
response. Instead responsiveness is stimulated by epitopes con-
The Components of Anti-Infection Defense...