3. Overview
• Immunity refers to protection against infections.
• The immune system is the collection of cells and molecules
that are responsible for defending the body against the
countless pathogens that individuals encounter.
• Defects in the immune system render individuals easy prey to
infections and are the cause of immunodeficiency diseases.
• But the immune system is itself capable of causing tissue
injury and disease, which are often referred to as
hypersensitivity disorders.
4. THE NORMAL IMMUNE RESPONSE
Defense against pathogens consists of two types of
reactions.
1. Innate immunity (also called natural, or native,
immunity) is mediated by cells and proteins that are
always present (hence the term innate), poised to
react against infectious pathogens.
• These mechanisms are called into action immediately
in response to infection, and thus provide the first line
of defense.
5. THE NORMAL IMMUNE RESPONSE
Many pathogens have evolved to resist innate
immunity, and protection against these infections
requires the more specialized and powerful
mechanisms of adaptive immunity (also called
acquired, or specific, immunity).
2. Adaptive immunity is normally silent and
responds (or adapts) to the presence of infectious
agents by generating potent mechanisms for
neutralizing and eliminating the pathogens.
6.
7. Innate Immunity
The innate immune system provides host defense by the
following two main reactions:
i) Inflammation.
• Cytokines-(regulatory proteins that regulate the cell of the immune
system) and products of complement activation, as well as
other mediators, are produced during innate immune
reactions and trigger the vascular and cellular
components of inflammation.
• The recruited leukocytes destroy pathogens and ingest
and eliminate damaged cells.
8. Innate Immunity
ii)Anti-viral defense.
• Type I interferons-(T-helper cells stimulate interferrons &
interferrons activate the other cells to phagocytize)
produced in response to viruses act on infected and
uninfected cells and activate enzymes that degrade viral
nucleic acids and inhibit viral replication.
• In addition to these defensive functions, the innate
immune system generates signals that stimulate the
subsequent, more powerful adaptive immune response.
9. 2. Adaptive Immunity
• The adaptive immune system consists of lymphocytes
and their products, including antibodies.
There are two types of adaptive immunity:
i) humoral immunity, mediated by soluble proteins
called antibodies that are produced by B lymphocytes
(also called B cells), and
ii) cell-mediated (or cellular) immunity, mediated by T
lymphocytes (also called T cells).
• Antibodies provide protection against extracellular
pathogens in the blood, mucosal surfaces, and tissues.
10. 2. Adaptive Immunity
• T lymphocytes are important in defense against
intracellular microbes.
• They work by either:
directly killing infected cells (accomplished by
cytotoxic T lymphocytes) or
by activating phagocytes to kill ingested microbes,
via the production of soluble protein mediators called
cytokines (made by helper T cells).
11.
12. Lymphoid Tissues
• The tissues of the immune system consist of:
1. The generative lymphoid organs (also called
primary, or central), in which T lymphocytes and B
lymphocytes mature and become competent to
respond to antigens.
2. These are the thymus where T-cells develop and the
bone marrow; the site of production of all blood
cells and where B-lymphocytes mature.
13. Lymphoid Tissues
The peripheral (or secondary) lymphoid organs, in
which adaptive immune responses to microbes are
initiated.
The peripheral lymphoid organs include:
• Lymph nodes
• Spleen
• The cutaneous and mucosal lymphoid systems are
located under the epithelia of the skin and the
gastrointestinal and respiratory tracts, respectively.
14. Lymphoid Tissues
• They respond to antigens that enter through
breaches in the epithelium.
• Pharyngeal tonsils and Peyer’s patches of
the intestine are two anatomically defined
mucosal lymphoid tissues.
• The large number of lymphocytes in mucosal
organs (second only to lymph nodes) reflects the
huge surface area of these organs.
15. HYPERSENSITIVITY : IMMUNOLOGICALLY MEDIATED
TISSUE INJURY
• Immune responses that normally are protective also
are capable of causing tissue injury.
• Injurious immune reactions are grouped under
hypersensitivity, and the resulting diseases are called
hypersensitivity diseases.
• This term originated from the idea that persons who
mount immune responses against an antigen are
sensitized to that antigen, so pathologic or excessive
reactions represent manifestations of a hypersensitive
state.
16. • Hypersensitivity reactions can be subdivided into
four types based on the principal immune
mechanism responsible for injury;
• three are variations on antibody-mediated injury,
and the fourth is T-cell mediated.
17. Types of Hypersensitivity Reactions
The main types of hypersensitivity reactions are as follows:
• A-anaphylactic/ immediate (type I) hypersensitivity,
often called allergy,
the injury is caused by T-helper(TH2) cells, immunoglobulin E(IgE)
antibodies and mast cells and other leukocytes.
Mast cells release mediators that act on blood vessels and smooth muscle
as well as cytokines that recruit and activate inflammatory cells.
• C-cytotoxic/Antibody-mediated disorders (type II hypersensitivity)
are caused by secreted IgG and IgM antibodies that bind to fixed tissue or
cell surface antigens.
Antibodies injure cells by promoting their phagocytosis or lysis and injure
tissues by inducing inflammation.
Antibodies also may interfere with cellular functions and cause disease
without cell or tissue injury.
18. Types of Hypersensitivity Reactions
• I-immune complex–mediated disorders (type III
hypersensitivity),
IgG and IgM antibodies bind antigens,
form antigen-antibody complexes that deposit in vascular beds
and induce inflammation.
The leukocytes that are recruited (neutrophils and monocytes)
produce tissue damage by release of lysosomal enzymes and generation
of toxic free radicals.
• D-delayed/T cell–mediated (type IV) hypersensitivity
disorders
are caused mainly by immune responses in which T
lymphocytes of the TH1 and TH17 subsets produce cytokines
that induce inflammation and activate neutrophils and
macrophages, which are responsible for tissue injury.
CD8+ CTLs also may contribute to injury by directly killing
host cells.
21. IMMUNODEFICIENCY SYNDROMES
• Immune deficiencies can be divided into:
1. Primary (or congenital)immunodeficiency disorders,
which are genetically determined, and
2. Secondary (or acquired) immunodeficiencies, which
may arise as complications of cancers, infections,
malnutrition, or side effects of immunosuppression,
irradiation, or chemotherapy for cancer and other
diseases.
• Immunodeficiencies are manifested clinically by
increased infections, which may be newly acquired or
reactivation of latent infections.
23. Summary
• Normal immune response include Innate and adaptive
immune response
• Innate immunity includes inflammation and antiviral
defense
• Adaptive immunity includes humoral and cellular
immunity
• Lymphoid tissues include bone marrow, thymus,
lymph nodes, Spleen, and the cutaneous and mucosal
lymphoid systems
24. Summary
• Hypersensitivity reactions include Anaphylactic
reaction, cytotoxic, immune-complex mediated and
delayed reactions
• Autoimmunity refers to immune reactions against self
(“auto”) antigens
• Immunodeficiency could be primary or secondary
