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The cells and molecules responsible for immunity constitute the
immune system, and their collective and coordinated response to
the introduction of foreign substances is called the immune
• Innate immunity (also called natural or native immunity) consists of
cellular and biochemical defense mechanisms that are in place.
• The principal components of innate immunity are
– (1) physical and chemical barriers, such as epithelia and antimicrobial
substances produced at epithelial surfaces;
– (2) phagocytic cells (neutrophils, macrophages) and NK (natural killer)
– (3) blood proteins, including members of the complement system and
other mediators of inflammation; and
– (4) proteins called cytokines that regulate and coordinate many of the
activities of the cells of innate immunity.
• Innate immunity provides the early lines of defense against
microbes and may not distinguish specifically differences between
• Because this form of immunity develops as a
response to infection and adapts to the
infection, it is called adaptive immunity.
• Specific immunity.
• Cellular Immunity (Macrophages, Antigen
Presenting Cells, T lymphocytes)
• Humoral immunity (B lymphocytes,
Plasmocytes/Plasma Cells and
Epithelia produce peptides
that have a natural antibiotic
- intraepithelial T
lymphocytes and the B-1
subset of B cells,
respectively, and these cells
may recognize and respond
to commonly encountered
Neutrophils and monocytes are recruited from the blood to sites of infection by binding to
adhesion molecules on endothelial cells and by chemoattractants produced in response to
Natural Killer Cells
A. NK cells kill host cells infected by intracellular microbes, thus eliminating
reservoirs of infection.
B. NK cells respond to IL-12 produced by macrophages and secrete IFN-γ, which
activates the macrophages to kill phagocytosed microbes.
NK cell activation is regulated by a balance between signals that are generated from activating
receptors and inhibitory receptors
The complement system consists
of several plasma proteins that
are activated by microbes and
promote destruction of the
microbes and inflammation.
Other Circulating Effector Proteins of
• Mannose-binding lectin, is a plasma protein that
functions as an opsonin.
• C-reactive protein is a plasma protein that typically
binds to bacterial phospholipids. It functions as an
• C-reactive protein is called an acute-phase reactant
because its plasma levels increase during the acute
stages of many infections.
• Coagulation factors are plasma proteins that mainly
function to prevent hemorrhage by forming a
thrombus at sites where blood vessel integrity is
Cytokines of Innate Immunity
• The cytokines of innate immunity recruit and
activate leukocytes and produce systemic
alterations, including increases in the
synthesis of effector cells and proteins that
potentiate antimicrobial responses.
Role of Innate Immunity in Stimulating Adaptive
A. Macrophages and dendritic cells
respond to phagocytosed (cellassociated) microbes, to
activate T lymphocytes
B. B lymphocytes recognize
microbial antigens by their
antigen receptors and recognize
complement system, to activate
the B cells.
Cellular Immunity is
mediated by T
called T cells).
Humoral immunity is
molecules in the
blood and mucosal
antibodies, that are
produced by cells
called B lymphocytes
Active and passive immunity.
Adaptive Immune Response
• Specificity Ensures that distinct antigens elicit specific
• Diversity Enables immune system to respond to a
large variety of antigens
• Memory Leads to enhanced responses to repeated
exposures to the same antigens
• Specialization Generates responses that are optimal
for defense against different types of microbes
• Self-limitation Allows immune system to respond to
newly encountered antigens
• Nonreactivity to self Prevents injury to the host
during responses to foreign antigens
The principal cells of the immune system are lymphocytes, antigen-presenting cells,
and effector cells.