Download Chapter_021

Chapter 21
Immune System
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 1
Introduction

The immune system protects against assaults
on the body

External assaults include microorganisms—
protozoans, bacteria, and viruses

Internal assaults—abnormal cells reproduce and
form tumors that may become cancerous and
spread
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 2
Organization of the Immune System

The immune system is continually at work patrolling and
protecting the body

Identification of cells and other particles

Self markers—molecules on the surface of human cells that are unique to
an individual, thus identifying the cell as “self” to the immune system

Non–self markers—molecules on the surface of foreign or abnormal cells
or particles and identify the particle as “non–self” to the immune system

Self-tolerance—the ability of our immune system to attack abnormal or
foreign cells but spare our own normal cells

Two major categories of immune mechanisms—innate
immunity and adaptive immunity (Figure 21-1; Table 21-1)

Innate immunity provides a general, nonspecific defense
against anything that is not “self”
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 3
Organization of the Immune System





Adaptive immunity acts as a specific defense against specific
threatening agents
Primary cells for innate immunity—epithelial barrier cells,
phagocytes (neutrophils, macrophages), and natural killer cells;
chemicals used in innate immunity—complement and interferon
Primary types of cells for adaptive immunity—lymphocytes called T
cells and B cells
Cytokines—any of several kinds of chemical released by cells to
promote innate and adaptive immune responses (examples:
interleukin, interferon, leukotriene)
Other chemicals (e.g., complement, other enzymes, and histamine)
also play regulatory roles in immunity
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 4
Innate Immunity (Table 21-2)

Species resistance—genetic characteristics
of an organism or species defends against
pathogens

Mechanical and chemical barriers—first line
of defense (Figure 21-2)

The internal environment of the body is protected
by a barrier formed by skin and mucous
membranes

Skin and mucous membranes provide additional
immune mechanisms—sebum, mucus, enzymes,
and hydrochloric acid in the stomach
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 5
Innate Immunity

Inflammation—second line of defense
(Figure 21-3)

Inflammatory response—tissue damage elicits
responses to counteract injury and promote normalcy
• Inflammation mediators include histamine, kinins,
prostaglandins, and related compounds (Figure 21-4)
• Chemotactic factors—substances that attract white blood
cells to the area in a process called chemotaxis
• Characteristic signs of inflammation—heat, redness, pain,
and swelling
• Systemic inflammation—occurs from a body-wide
inflammatory response
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 6
Innate Immunity

Inflammation—second line of defense (cont.)

Phagocytosis—ingestion and destruction of
microorganisms or other small particles by
phagocytes (Figure 21-7)
• Antigen-presenting cells (APCs)—phagocytes that ingest
foreign particles, isolate protein segments (peptides), and
display them as antigens on their surfaces to trigger an
immune response when recognized by a specific (adaptive)
immune cell
• Chemotaxis—chemical attraction of cells to the source of
the chemical attractant (Figure 21-6)
• Diapedesis—process by which immune cells squeeze
through the wall of a blood vessel to get to the site of
injury/infection (Figure 21-5)
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 7
Innate Immunity

Phagocytosis (cont.)
• Neutrophil—most numerous type of phagocyte; usually
first to arrive at site of injury; migrates out of bloodstream
during diapedesis; forms pus
• Macrophages (Table 21-3)

Phagocytic monocytes grow larger after migrating from
bloodstream

Dendritic cell (DC)—type of macrophage with long
branches or extensions (Figure 21-8)

Examples are histiocytes in connective tissue, microglia in
nervous system, and Kupffer cells in liver
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 8
Innate Immunity

Natural killer cells—lymphocytes that kill tumor cells and cells
infected by viruses (Figure 21-9)

Method of recognizing abnormal or non–self cells—target cell is killed if
killer-inhibiting receptor on NK cell does not bind to a proper MHC
surface protein
 Method of killing cells—lysing cells by damaging plasma membranes


Interferon (INF)—protein synthesized and released into the
circulation by certain cells if invaded by viruses to signal other,
nearby cells to enter a protective antiviral state
Complement—group of enzymes that produce a cascade
of reactions resulting in a variety of immune responses
(Figure 21-10)



Lyse cells when activated by either adaptive or innate mechanisms
Opsinization—mark cells for destruction by phagocytes
Variety of other immune responses
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 9
Overview of Adaptive Immunity

Adaptive immunity is part of the third line of defense consisting
of lymphocytes—two different classes of a type of white blood
cell (Figure 21-11)

Two classes of lymphocytes (Figure 21-12)—B lymphocytes
(B cells) and T lymphocytes (T cells)

Subsets of lymphocytes are defined by the cluster designation
(CD) surface markers that the cells carry, for example, CD4
and CD8 cells

Lymphocytes flow through the bloodstream, become
distributed in tissues, and return to the bloodstream in a
continuous recirculation

B-cell mechanisms—antibody-mediated immunity (humoral
immunity); produce antibodies that attack pathogens
(Figure 21-13)
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 10
Overview of Adaptive Immunity

T cells attack pathogens more directly—classified
as cell-mediated immunity (cellular immunity)

Lymphocytes have protein markers on their
surfaces that are named using the CD system

Activation of lymphocytes requires two stimuli:
a specific antigen and activating chemicals
(Figure 21-14)

Lymphocytes are densest where they develop—
in bone marrow, thymus gland, lymph nodes,
and spleen (Figure 21-15)
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 11
B Cells and
Antibody-Mediated Immunity

B cells develop in two stages:

Pre-B cells develop by a few months of age

The second stage occurs in lymph nodes and spleen—activation of a
naïve B cell after it binds a specific antigen

B cells serve as ancestors to antibody-secreting plasma cells

Antibodies—proteins (immunoglobulins) secreted by activated B cell
(Figure 21-16)

An antibody molecule consists of two heavy and two light polypeptide
chains; each molecule has two antigen-binding sites and two
complement-binding sites (Figure 21-17)

Babies are born with different clones of B cells in bone marrow, lymph
nodes, and spleen; cells of the clone synthesize a specific antibody
with a sequence of amino acids in its variable region that is different
from the sequence synthesized by other clones
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 12
B Cells and
Antibody-Mediated Immunity


Five classes of antibodies (Figure 21-18)—
immunoglobulins M, G, A, E, and D

IgM—antibody that naïve B cells synthesize and insert into their
own plasma membranes; it is the predominant class produced
after initial contact with an antigen

IgG—makes up 75% of antibodies in the blood; predominant
antibody of the secondary antibody response

IgA—major class of antibody in the mucous membranes and in
saliva and tears

IgE—small amount; produces harmful effects such as allergies

IgD—small amount in blood; precise function unknown
Antibody molecules produce antibody-mediated immunity
(humoral immunity)—within plasma
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 13
B Cells and
Antibody-Mediated Immunity

Antibodies resist disease first by recognizing foreign or abnormal
substances (Figure 21-19)


Epitopes bind to an antibody molecule’s antigen-binding sites, forming
an antigen-antibody complex that may produce several effects
(Figure 21-20)
Complement—a component of blood plasma consisting of several
protein compounds

Complement kills foreign cells by cytolysis or apoptosis
(Figures 21-21 and 21-22)

Complement causes vasodilation, enhances phagocytosis, and other
functions

Complement activity can also be initiated by innate immune mechanisms
• Formation by innate immunity is called the alternate pathway
• Complement protein 3—activated without antigen stimulation—produces full
complement effect by binding to bacteria or viruses in presence of properdin
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 14
B Cells and
Antibody-Mediated Immunity

Primary and secondary responses (Figure 21-23)

Primary response—initial encounter with a specific antigen triggers the
formation and release of specific antibodies that reaches its peak in a
few days
 Secondary response—a later encounter wit the same antigen triggers a
much quicker response; B memory cells rapidly divide, producing more
plasma cells and thus more antibodies

Clonal selection theory (Figure 21-24)

The body contains many diverse clones of cells, each committed by its
genes to synthesize a different antibody
 When an antigen enters the body, it selects the clone whose cells are
synthesizing its antibody and stimulates them to proliferate and create
more antibody
 The clones selected by antigens consist of lymphocytes and are selected
according to the shape of antigen receptors on the lymphocyte’s plasma
membrane
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 15
T Cells and Cell-Mediated Immunity


T cells—lymphocytes that go through the thymus gland before
migrating to the lymph nodes and spleen

Pre-T cells develop into thymocytes while in thymus

Thymocytes stream into blood and are carried to T-dependent zones in
spleen and lymph nodes
Activation of T cells

T cells display antigen receptors on their surface membranes that are
similar to antibodies

A T cell is activated when an antigen (presented by an APC) binds to
its receptors, causing it to divide repeatedly to form a clone of identical
T cells (Figure 21-25)
• Cells of the clone differentiate into effector T cells and memory T cells
• Effector T cells go to site where antigen entered, bind to antigens, and begin
their attack
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 16
T Cells and Cell-Mediated Immunity


Cytotoxic T cells—T cells release lymphotoxin to kill
cells (Figure 21-26)
Helper T cells (TH cells) —regulate the function of
B cells, T cells, phagocytes, and other leukocytes
(Figure 21-27)

Suppressor T cells—regulatory T cells that
suppress lymphocyte function, thus regulating
immunity and promoting self-tolerance

T cells function to produce cell-mediated immunity
and help to regulate adaptive immunity in general
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 17
Types of Adaptive Immunity
(Table 21-4)


Innate immunity (inborn or inherited immunity)—genetic
mechanisms put innate immune mechanisms in place during
development in the womb
Adaptive or acquired immunity; resistance developed after
birth; two types:

Natural immunity results from nondeliberate exposure to antigens
 Artificial immunity results from deliberate exposure to antigens, called
immunization

Natural and artificial immunity may be active or passive

Active immunity—when the immune system responds to a harmful
agent regardless of whether it was natural or artificial; lasts longer
than passive
 Passive immunity—developed when immunity from another
individual is transferred to an individual who was not previously
immune; it is temporary but provides immediate protection
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 18
Summary of Adaptive Immunity


Adaptive immunity is specific immunity—targeting specific
antigens
Adaptive immunity involves two classes of lymphocyte: B cells
and T cells (Figure 21-27)

B cells—antibody-mediated (humoral) immunity
 T cells—cell-mediated (cellular) immunity

Adaptive immunity occurs in a series of stages (Figure 21-28)






Recognition of antigen
Activation of lymphocytes
Effector phase (immune attack)
Decline of antigen causes lymphocyte death (homeostatic balance)
Memory cells remain for later response if needed
B cells and T cells work together in a coordinated system of
adaptive immunity (Figure 21-29)
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 19
The Big Picture:
Immune System and the Whole Body

Immune system regulated to some degree by
the nervous and endocrine systems

Agents of the immune system include blood
cells, skin cells, mucosal cells, brain cells,
liver cells, and other types of cells and their
secretions
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 20