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Immune system
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© Zanichelli editore 2016
Three layers of defense
The immune system protects the body with three layers of
defense:
• physical and chemical barriers, like the skin and
chemicals produced by mucous membranes (tears, saliva,
acids in the stomach and vagina, ear wax);
• innate immune system;
• adaptive immune system.
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© Zanichelli editore 2016
Cells of the innate immune system
The innate immune system (non-specific immune system)
consists of cells and chemical signals that have the ability to:
recognize pathogens through antigen receptors;
o call other defense cells;
o destroy pathogens and infected cells.
o
Cells of the innate immune system:
• phagocytes (neutrophils, macrophages, dendritic
cells), that engulf and kill pathogens;
• natural killer cells that kill infected or cancer cells.
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Inflammation and fever
Inflammation is the immune system’s response to infection.
The release of chemical signals (cytokines and histamine)
attracts phagocytes and other cells of the immune system to
an infection site.
Cytokines induce fever which stimulates immune response
and reduces the growth rate of many pathogens.
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© Zanichelli editore 2016
Cells of the adaptive immune system /1
B lymphocytes are the cells involved in humoral immunity
(mediated by antibodies). They are produced by bone marrow
wherein they become mature.
They release antibodies in response to the presence of an
antigen.
antibody
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antigen
Y
B lymphocyte
pathogen
© Zanichelli editore 2016
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Cells of the adaptive immune system /2
T lymphocytes are the cells involved in cell-mediated
immunity. They are produced by the bone marrow, but
mature in the thymus.
They fight pathogens by destroying infected cells.
infected cell
pathogens
T lymphocyte
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Memory cells
During the first response against a pathogen, lymphocytes
respond to the antigen and fight the pathogen.
In case of a second encounter with the same antigen,
memory cells produced during the first response activate the
secondary response which is faster and more intense.
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Plasma cells and antibodies
Plasma cells are white blood cells that produce a large
number of antibodies.
Each antibody has a unique structure which allows it to
recognize a specific antigen. Antibodies can be released by B
lymphocytes and freely move in the blood stream or they can
attach to the B lymphocyte membrane.
Y
Y
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Immunization
Long-term immunity against a pathogen can be obtained:
• through previous exposure to the natural pathogen;
• with immunization (vaccination), through the
administration of a vaccine. Vaccines contain the attenuated
– or harmless form – of a pathogen.
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Diseases of the immune system
Autoimmune diseases occur when the immune system in
not working properly by attacking its own structures and
damaging tissues and organs. Type I diabetes is an
autoimmune disease.
In immunodeficiency disorders, the immune system
correctly recognizes pathogens, but the response is
insufficient. AIDS is an acquired immunodeficiency, caused
by the HIV virus.
Allergies are anomalous immune responses against
harmless substances.
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© Zanichelli editore 2016