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NATIONAL CHENG KUNG UNIVERSITY MEDICAL COLLEGE
DEPARTMENT OF MEDICAL TECHNOLOGY
Monday, September 17, 2007, 1:10-3:00 p.m.
INTRODUCTION TO THE IMMUNE SYSTEM
Yee-Shin Lin, Ph.D.
A. BACKGROUND
The immune system is the second line of defense against invasion by foreign organisms. [The integument is the
first] [Fig. 1.5]. In addition, the immune system polices and destroys spontaneously arising tumor cells in the
body. Sounds simple, doesn’t it? However, the immune system must first make the distinction between normal
tissues (self) and abnormal or foreign (non-self) tissues. The molecular determinants recognized by the immune
system are called epitopes. One or more epitopes are collectively called an antigen [Fig. 1.9].
Reading: Male, Brostoff, Roth & Roitt [chapter 1]
B. LECTURE SUMMARY
ANTIGENS -- SELF vs. NON-SELF
The immune system distinguishes what is SELF from what is NON-SELF, the "us" and the "them", on a cellular
and molecular level. It recognizes bacteria, viruses, protozoa, fungi, etc. Immunologists use the collective term
antigen to identify any substance recognized by the immune system. There are 3 distinct subsets of antigens:
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
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Immunogen: substance recognized by the immune system that provokes an immune response.
Hapten: a substance recognized by the immune system, but that does not, by itself, induce an immune
response.
Tolerogen: a substance that paralyzes and doesn’t allow the induction of the immune response or a
substance that specifically turns off an immune response [chapter 19].
Although the immune system does not respond to an isolated hapten, it will respond to a hapten physically
coupled to an immunogen. The immunogen to which hapten is coupled is termed a carrier. When the immune
system recognizes a hapten-immunogen (or hapten-carrier) complex, an immune response occurs directed
against both hapten and carrier.
Injection of an animal with an immunogen, resulting in the elicitation of an immune response, is called
immunization.
What does the immune system recognize? Most immunogens contain multiple antigenic determinants called
epitopes. An epitope is the basic minimal structure recognized by the immune system. Most immunogens are
composed of a variety of epitopes, whereas most haptens comprise a single epitope. Some general
characteristics of IMMUNOGEN:
1. must be recognized as non-self
2. physical properties
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
size -- usually proteins > 10,000 daltons (proteins and carbohydrates best, nucleic acids can also be
immunogenic. Lipids in most cases are not.)
 complexity -- more complex the molecule --> more potential epitopes --> more chance for these epitopes
to be recognized as foreign
 conformation and accessibility -- epitopes must be "seen" by and be available to the immune system
(some tissues of the body are not normally "seen" by the immune system, e.g. anterior chamber of the
eye, brain, etc.)
 charge -- does not appear to be important
3. chemical properties -- the immunogen, in most cases, has to be subject to enzymatic cleavage by
phagocytes
In the immune system, phagocytes ingest foreign materials, enzymatically cleave them, and display them on
their cell membranes. This mechanism is called antigen processing and is usually necessary for the induction
of an immune response.
The induction of an immune response is quite complex, involving the interaction of several different types of
cells. Immune responses are also determined by the genetic characteristics of the individual [will be detailed in
chapter 11].
INNATE vs. ADAPTIVE IMMUNITY
The immune response is mediated by a variety of molecules and cells, and can be broadly divided into two
functional divisions, namely the innate immunity and the adaptive immunity [p. 8]. The components of the
immune system are listed in Fig. 1.1.
INNATE IMMUNITY [will be detailed in chapter 6]
If the body’s exterior barrier fails to prevent the invasion of the infectious agents, the innate immunity will serve
as a first line of defense against these infectious agents.
The cells and molecules involved in the innate immunity are as follows:
1. Phagocytes: Their function is to engulf foreign materials, internalize them and destroy them. The
phagocytes include:
 polymorphonuclear neutrophils (PMN): short-lived cells
 monocytes/macrophages (mononuclear phagocyte system): long-lived cells [Fig. 1.2]
2. Natural killer (NK) cells (Large granular lymphocytes, LGLs): They are able to kill viral-infected and
tumor cells to which they have not been previously sensitized [Fig. 1.8].
3.
Soluble factors
 acute phase proteins, e.g. C-reactive protein (CRP): increase 2 to 100X during infection, and act as an
opsonin to promote the binding of complement which facilitates phagocytosis [p. 6].
 complement: consists of a group of about 20 serum proteins which cause direct lysis of bacteria,
chemotaxis and opsonization [Fig. 1.3 and chapter 4].
 cytokines, e.g. interferons: limit the spread of viral infection and enhance NK cell activity [Fig. 1.4].
ADAPTIVE IMMUNITY
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Two key features of adaptive immune response are specificity and memory [p. 10]. This is accomplished by
either B cells which produce antibodies or T cells which include TH, Tc (CTL) [Fig. 1.8]. These are referred to
as two arms of the immune system: humoral immunity and cell-mediated immunity. These two systems of
immunity act in concert to provide protection for the host.
Specificity: antigens are recognized by specific receptors on lymphocytes. For B cells: cell surface antibody; for
T cells: T cell receptor (TCR).
 Each antibody binds to a restricted part of the antigen called an epitope [Fig. 1.9].
Antibody acts as an adaptor (opsonin) between the infectious agent and phagocyte [Fig. 1.10]. Antibody
also causes the activation of complement, the infectious agent often has both antibody and C3b bound to
the surface. The attachment and phagocytosis are therefore greatly enhanced [Fig. 1.11].
 T cells recognize the antigenic peptide-MHC molecule complex [Fig. 1.12].
CLONAL SELECTION
Concept: The immune system has a specific cell for each the millions of possible epitopes. Scientists estimate
that the antigenic universe contains between 106 and 107 epitopes. This means that there are at least 106 to 107
different B or T cells. Lymphocytes are very specialized, whenever the term antigen specific is used, this means
that there is a cell (B or T cell) resident in the immune system for each of the 106 and 107 epitopes! [Fig. 1.13]
DEFENCES AGAINST EXTRACELLULAR AND INTRACELLULAR PATHOGENS: [Fig. 1.6, 1.15]
VACCINATION [Fig. 1.16] -- detailed in Tetanus Immunity Workbook
INFLAMMATION [This process is described more fully in chapter 6]
Three major things occur:
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Increased blood supply to the infected area.
Increased capillary permeability.
Leucocytes migrate out of the venules into the surrounding tissues. This process includes pavementing,
diapedesis, chemotaxis, and phagocytosis. [Fig. 1.18, 1.14].
IMMUNOPATHOLOGY: Hypersensitivity, immunodeficiency, and autoimmunity [Fig. 1.19]
C. ImmunObjectives -- a self assessment
1. Define:
 self/non-self
 immunologic memory and specificity
 opsonization
 vaccination
 inflammation
2. Define antigen, distinguish it from immunogen. What is an epitope?
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3. Describe the soluble factors and cells involved in innate and adaptive immunity, respectively, and the
function of each factor and cell.
4. Describe chemotaxis and phagocytosis that are involved in inflammation process.
5. Describe the difference between immune responses to extracellular and intracellular pathogens.
6. Describe how the immune system may itself cause diseases to a person.