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The Immune system
Biology II: Form and Function
The Immune System
• Nonspecific defense mechanisms
– Physical barriers (skin, mucous
membranes
– Internal defense 1
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Phagocytic neutrophils
Macrophage monocytes
Natural killer cells
The antimicrobial protein complement
• Specific defense mechanisms (the
Immune system)
– T- and B-Lymphocytes, antibodies
Physical barriers
• Integument
– is acidic (pH 3-5)
– Flakes away, taking away particles
– Subcutaneous adipose tissue helps cushion
physical contact
– Sweat contain lysozyme
• Mucous
– prevents microbes from reaching sensitive
epithelial tissue
– Washes surface of dust
– Viscous natures traps particles (cilia)
Nonspecific defenses: the front line
– Monocytes are transformed into macrophages that
ingest microbes and fuse them with lysosomes (highly
oxidative)
– Neutrophils can also act as macrophages, but more
commonly release poisons that work on a large scale
– Natural killer cells destroy already infected cells by
releasing molecules of perforin that breach the target
cell. Sudden influx of water lyses foreign cell
– The protein complement system aggregates to form
membrane attack complexes also to breach target cells,
as well as releasing proteins that inhibit virus
transcription (also a, b, g interferons)
The inflammatory/temperature response
– Invasion of foreign material causes injured cells
to release chemical alarm signals (e.g. histamine)
– Chemical alarm signals promote vasodilation
– Vasodilation and increased permeability of
capillaries causes edema (tissue swelling)
– Increased permeability allows macrophages to
cross over into infected site
– Macrophages release interleukin-1, causing body
to raise temperature (fever), which causes mild
anemia
– Localized infections can be serious enough to
cause systemic response
The Immune System
– The immune system responds to the presence of
antigens, specific complex protein molecules associated
with bacteria, viruses and other foreign bodies
– Humoral immune response produces antibodies (Blymphocytes)
– Cell-mediated immunity (T-lymphocytes)
– Specific defense against microbial invasion: response
elicited by antigens
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Specificity: Specialized lymphocytes produce antibodies
Diversity (over 109 different types of antibody)
Memory (acquired/passive immunity)
Self/nonself recognition (defense is specific against antigen.
Failure to recognize healthy, non-antigen tissue causes auto
immune diseases (e.g. lupus))
Humoral Response/Antibodies
– Produced by B-Lymphocytes (from bone marrow)
– Antibodies belong to a group of proteins called
Immunoglobulins (Ig)
– General structure is “Y” shaped, made of two light
and two heavy polypeptide chains
– Heavy chains specify type of antibody (IgM, IgG,
IgA, etc.)
– Light chains responsible for antigen specificity
• Initial encounter between B-lymphocyte
with specific antibody and antigen
causes clonal selection: division of Bcells to
– Memory cells
– Plasma cells that produce more antibodies
that flag antigen for destruction by
• aggregation of protein complement system
• Macrophage activity
Cell-mediated Response
• T-cells have similar receptor sites to B-cells
• Response to binding between recognition
site and antigen is production of:
– Cytotoxic T-cells (Tc) attack cells labeled for
destruction by helper T-cells
– Helper T-cells (Th), produce cytokines
(interleukin) to regulate immune response
When things go wrong (1)
– Acquired Immuno-Deficiency Syndrome
(AIDS) is represented by a breakdown of
the immune response system, caused by
the virus HIV (Human-Immunodeficiency
Virus)
• HIV infects T-cells until entire population of
certain types are destroyed
• Infected T-cells secrete chemical that block
defense by other T-cells
• HIV blocks recognition sites so that infected
cells cannot be destroyed by immune system
When things go wrong (2)
– Antigen shifting causes acquired immunity
to fails by changing the ‘face’ of the antigen
• Seen in Malaria, Bilharzia
– Autoimmune diseases caused by failure of
immune system to recognize self (causes
inflammation and organ damage)
– Allergies caused by abnormal response of
• B-cells to allergen (immediate hypersensitivity),
producing a histamine reaction
• T-cells (delayed hypersensitivity)