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Adaptive Immunity: Specific
Defenses of the Host
Chapter 17
I. Introduction to Specific Defenses of the Host
• A. Innate immunity (resistance) – An individual’s genetically
predetermined resistance to certain diseases. Affected by
sex, age, nutritional status, and general health (previously
discussed in Chap 16).
• B. Acquired (adaptive) immunity – Ability of the body to
counteract specific microbes or foreign substances. Immunity
results from the production of protein antibodies and
specialized lymphocytes that target a specific antigen that
caused their formation. The outcome is destruction or
inactivation of the antigen for current or future exposures.
The immune system is required for survival, but if misdirected
toward self antigens it can be very destructive to the host.
Types of Acquired Immunity - Figure 17.16
I. Naturally acquired active
II.Naturally acquired passive
III.Artificially acquired active and
vaccination
IV.Artificially acquired passive and gamma
globulin fig 17.17
II. Types of Acquired Immunity
Figure 17.17
III. The Duality of the Immune System
• A. Mediated by lymphocytes via the lymphatic system.
• Fluid circulates between interstitial fluid, lymphatic
vessels and nodes, then returned to the blood.
Antigens in this fluid are processed by the immune
system. Fig 16.5
• B. Humoral (Antibody Mediated) Immunity
– Protein Antibodies are produced by B Cells (B
Lymphocytes) and secreted into extracellular fluids.
• Defends against bacteria, viruses, and toxins in body fluids
• C. Cell Mediated by specialized lymphocytes
– Involves T-cells (T lymphocytes) and no antibodies. Act
against foreign organisms or tissues. Regulates the
activation and proliferation of other immune system cells.
• Most effective against intracellular bacteria and viruses, fungi,
multicellular parasites, cancer cells, and transplanted tissue.
IV. Antigens and Antibodies
• A. Antigens – Substances that cause the body to produce
specific antibodies and/or sensitized T-cells.
– 1. Foreign – Not part of body chemistry. Nonself vs self
recognition so the immune system does not attack the
host’s own tissues.
– 2. Proteins or large polysaccharides with a molecular
weight greater than 10,000. Combinations can be
antigenic: nucleoproteins, lipoproteins, glycoproteins.
Most components of an organism are antigenic.
– 3. Antibodies are formed against specific regions on the
surface of an antigen called ‘antigenic determinant’
(epitope). Fig 17.1
– 4. Hapten is a low molecular weight compound that
cannot induce antibody response by itself, but can if
combined with another molecule (often a host protein).
• E.g. penicillin fig 17.2
Epitopes: antigenic determinanants
Figure 17.1
Haptens
Figure 17.2
IV. Antigens and Antibodies
• B. Antibodies –
Proteins made in
response to antigens
that can only recognize
and bind with the
antigen that stimulated
their formation.
– 1. Structure – Typically
Y-shaped with two light
and two heavy chains.
Variable region provides
for specific antigen
recognition and binding.
IV. Antigens and Antibodies
IV. Antigens and Antibodies: Antibodies
• 2. Immunoglobulin Classes
– IgG (80% of blood antibodies), crosses placenta
– IgM (5-10% of serum antibodies), used to detect
initial infection
– IgA (primarily in mucous and secretions for
localized protection), especially important in
respiratory and GI tract.
– IgD (antigen receptors on B cells for initiation of
immune response)
– IgE (bind to mast cells in tissue), mediates
allergies, lysis of parasitic worms.
IV. Antigens and Antibodies: Antibodies
V. B Cells and Humoral Immunity
• A. Bone marrow stem
cells give rise to B cells
that migrate to
lymphoid organs where
they come in contact
with antigen, which
they recognize, via
antibody on surface of
their cells.
V. B Cells and Humoral Immunity
• 1. The antigen is processed and presented with a
MHC class II site on the surface of the B cell where
it can be recognized by a TH cell via its specific
receptor.
– The MHC is a self identity marker that prevents the host
from making antibodies harmful to itself.
– Most antigens elicit this type of response, called a T
dependent antigen response.
Activation of B cells to produce antibodies Figure 17.4 - Overview
V. B Cells and Humoral Immunity
• 2. B cells go through a
process of producing a
clone of plasma cells
(antibody producing
cells) and memory cells.
• An individual may
recognize about 1015
different antigens
determined by their
genetic makeup.
V. B Cells and Humoral Immunity
• B. Antigen-Antibody Binding and its results
– Agglutination, Opsonization, Neutralization,
Complement activation, Inflammation, Antibodydependent cell-mediated cytotoxicity
Figure 17.14 - Overview
V. B Cells and Humoral Immunity
• C.
Immunological Memory
– 1. Primary: gradual, several
days with no antibody, then
IgM, then IgG. Peak titers at
10-17 days.
– 2. Secondary Immune
Response: quick via memory
cells that persist for years,
primarily IgG. Some antibody
already present followed by
peak in 2-7 days.
– 3. T cell memory is similar
and necessary for self vs
nonself recognition
The primary and secondary immune response to an antigen
Figure 17.15
VI. T Cells and Cell-Mediated Immunity
• A. Chemical Messengers of
Immune Cells
– 1. Cytokines – Chemical way in
which cells communicate
– Example: Interleukin 2 (IL-2)
stimulates antigen stimulated T
helper cells and B cells,
activates T cytotoxic cells and
natural killer cells.
VI. T Cells and Cell-Mediated Immunity
• B. Cellular Components of Immunity
– 1. Lymphocytes produced in bone marrow
migrate to thymus, where under the influence of
thymic hormones, differentiate into T-cells.
– In response to antigen, effector T-cells are
stimulated to be produced. These cells proliferate
and carry out cell mediated immunity.
– The body’s ability to make new T cells decreases
with age beginning in late adolescence, so the
immune system is relatively weak in old age.
Memory cells remain to provide protection.
Differentiation of T cells and B cells
Figure 17.8
VI. T Cells and Cell-Mediated Immunity
• 2. Types of T cells : APC and T cell
– Classified according to function and cell surface receptors
(glycoproteins: clusters of differentiation (CD)).
– Antigen presenting cell (APC) = macrophages or
dendritic cells.
• Self molecule on surface = Major histocompatibility
complex (MHC).
• Is not a T cell, but is necessary to process antigen to
present to T cell
• A T cell only recognizes the antigen if presented with an
MHC self surface (Class II) marker close to it that is
unique to that person as defined in their genetic code.
The immune system uses this to determine self from
nonself.
A dendritic cell
Figure 17.12
Acivated macrophages
Figure 17.13
VI. T Cells and Cell-Mediated Immunity
• 2. Types of T cells
– T-helper cell (TH, CD4), T-cytotoxic cell (TC, CD8)
• Antigen must be presented by APC to T helper cell. IL-1
secreted by APC to stimulate TH cell. fig 17.9
• Antigen must be closely associated with MHC for
successful recognition by T cell. IL-2 secreted by TH
cells to stimulate and differentiate more T and B cells.
Amplifies response.
• TC cells attack whole cells (e.g. cancer cells, virus
infected cells where antibodies can’t get at them) with
surface antigen and MHC class I markers.
– Class I markers are found on nucleated host cells. So, these
cells can attack host cells that have an altered surface
antigen, such as tumor cells and virus infected cells. Fig 17.10
Activation of CD4 helper T cell by exogenousFigure
antigen
17.9 - Overview (1 of 4)
Activation of CD8 cytotoxic T cell by endogenous antigen
Figure 17.10 - Overview
VI. T Cells and Cell-Mediated Immunity
• 2. Types of T cells
– Regulatory T cell (TR ) – Turns off (suppresses) the
immune response when antigen no longer
present.
• May be involved with regulation of tissue rejection and
auto immune reactions.
VI. T Cells and Cell-Mediated Immunity
C. Interrelationship of Cell-Mediated and Humoral Immunity. Fig 17.18