the immune system chapter 51. 2 introduction vertebrates have three levels of defenses -1. the...
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The Immune System
Chapter 51
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Introduction
Vertebrates have three levels of defenses
-1. The Integumentary System
-Skin and mucous membranes provide first line of defense
-2. Nonspecific (innate) Immune System
-Acts very rapidly after onset of infection
-3. Specific Immune System
-Eliminates microbes that escaped the second line of defense
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Skin
The skin is the largest organ of the body
-Provides a nearly impenetrable barrier, reinforced with chemical weapons
-Oil & sweat glands give skin a pH of 3-5
-Lysozyme breaks bacterial cell walls
-Also contains many normal flora
-Non-pathogenic microorganisms that out-compete pathogenic ones
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Skin
The skin is composed of three layers
-Epidermis = 10-30 cells thick
-Stratum corneum – Outermost layer; cells shed continuously
-Stratum spinosum – Middle layer-Stratum basale – Innermost layer;
cells actively dividing-Contains keratin, which makes skin tough and water-resistant
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Skin
-Dermis = Contains connective tissue and is 15-40 times thicker than epidermis
-Provides structural support for epidermis, and matrix for blood vessels, muscles and nerve endings
-Subcutaneous layer = Contains mainly adipose (fat) cells
-Acts as shock absorbers and insulators
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Mucosal Epithelial Surfaces
The digestive, respiratory and urogenital tracts are lined by mucous membranes-Cells secrete mucus which traps microbes
Digestive tract -Salivary lysozyme; acidic stomach
Respiratory tract -Ciliary action
Urogenital tract-Acidic urine
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Nonspecific Immunity
The nonspecific or innate immune system consists of cellular and chemical devices that respond to any microbial infection
-The response is quite rapid
Among the most important defenses are three types of leukocytes (white blood cells)
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Leukocytes
Macrophages
-Large, irregularly shaped cells
-Kill microbes by phagocytosis
-Mature from monocytes that enter tissues from the blood
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Leukocytes
Neutrophils
-The most abundant circulating leukocytes
-First to appear at site of damage/infection
-Kill microbes by phagocytosis
Natural killer (NK) cells
-Destroy pathogen-infected and cancer cells by programmed cell death or apoptosis
-Produce perforins and granzymes
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Leukocytes
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Leukocytes (Cont.)
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The Inflammatory Response
Inflammation involves several body systems
-Injured cells release chemical alarms, including histamine and prostaglandins
-Cause nearby blood vessels to dilate and increase in permeability
-Promote phagocyte accumulation
-Hallmark signs = Redness, warmth, swelling, pain, and potential loss of function
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The Inflammatory Response
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The Inflammatory Response
Inflammation is accompanied by an acute phase response, manifested by fever
-Macrophages release interleukin-1
-Causes hypothalamus to raise body temperature
-Promotes activity of phagocytes, while impeding microbial growth
-However, very high fevers are hazardous as they may denature critical enzymes
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Complement
The complement system consists of about 30 different proteins that circulate in the blood in an inactive form
-Upon pathogen encounter, a cascade of activation occurs
-Some proteins aggregate to form a membrane attack complex (MAC) on surface of pathogen
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Complement
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Complement
Other functions of complement proteins
-C3b coats surface of invading pathogens, thereby enhancing their phagocytosis
-Some stimulate the release of histamine from mast cells and basophils
-Some attract more phagocytes to the area of infection
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Interferon
Interferons (IFN) are proteins that play a key role in body defense
-Three major types: IFN-, IFN-, IFN-
-IFN- and IFN-are produced by almost all body cells in response to viral infection
-Induce degradation of viral RNA
-IFN- is produced only by T-lymphocytes and natural killer cells
-Protects from infection and cancer
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The Specific Immune System
The scientific study of immunity began with Edward Jenner in 1796
-Observed that milkmaids who had cowpox rarely experienced smallpox
-Inoculated individuals with fluid from cowpox vesicles to protect them from smallpox
-Vaccination
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The Specific Immune System
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The four characteristics of the specific, or adaptive, immune response are:
-1. Specificity
-2. Diversity
-3. Memory
-4. Ability to distinguish self from non-self
The Specific Immune System
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Antigens
An antigen is a molecule that provokes a specific immune response
-May be components of microorganisms or proteins/glycoproteins found on surface of red blood cells or transplanted tissue cells
A single protein may have many different antigenic determinants or epitopes
-Each can stimulate a distinct immune response
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Antigens
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Lymphocytes
Lymphocytes are leukocytes with surface receptors for antigenic determinants
-Direct an immune response against either the antigen or the cell that carries it
When a naïve lymphocyte binds a specific antigen for the first time, it gets activated by a process called clonal selection
-Produces a clone of cells: some respond immediately, others are memory cells
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Lymphocytes
B lymphocytes or B cells
-Respond to antigens by secreting antibodies or immunoglobulins (Ig)
-Participate in humoral immunity
T lymphocytes or T cells
-Regulate other immune cells or directly attack cells that carry specific antigens
-Participate in cell-mediated immunity
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Acquisition of Specific Immunity
Immunity can be acquired in two ways
-Active immunity results from activation of an individual’s own lymphocytes
-Pathogen infection or vaccination
-Passive immunity results from obtaining another individual’s antibodies
-Transfer of maternal antibodies across placenta
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Hematopoiesis
All blood cells are derived from hematopoietic stem cells through hematopoiesis
-A lymphoid progenitor gives rise to lymphocytes and natural killer cells
-A myeloid progenitor gives rise to all other white blood cells, plus RBCs and platelets
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Organs of the Immune System
Organs of the immune system consist of the:
-Primary lymphoid organs
-Bone marrow and thymus
-Secondary lymphoid organs
-Lymph nodes, spleen, and mucosal-associated lymphoid tissue (MALT)
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Primary Lymphoid Organs
The bone marrow is site of B cell maturation
-Each B cell has about 105 Ig molecules on its surface, all with the same specificity
-However, different B cells will have different specificities
-B cells recognize epitopes directly
-Any lymphocytes that are likely to bind to self-antigens undergo apoptosis
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Primary Lymphoid Organs
The thymus is the site of T cell maturation
-Each T cell has about 105 identical T-cell receptors, or TCRs on its surface
-Recognize epitopes only if they are combined with major
histocompatibility complex (MHC) peptides
-Lymphocytes that cannot bind MHCs, or that bind self-MHC/self-peptide too tightly undergo apoptosis
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Secondary Lymphoid Organs
The locations of these organs promote the filtering of antigens that enter any part of an individual’s body
-Mature but naïve B and T cells become activated in the lymph nodes
-The spleen is site of immune responses to antigens found mainly in the blood
-Mucosal-associated lymphoid tissue (MALT) include the tonsils and appendix
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T Cells
T lymphocytes are of two types:
-Cytotoxic T cells (Tc)
-CD8+ cells
-Helper T cells (TH)
-CD4+ cells
-Distinguished by type of MHC markers recognized and roles after activation
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T Cells
In humans, the MHC complex is also termed human leukocyte antigens (HLAs)
-Markers that distinguish self from nonself
-MHC class I = Found on all nucleated cells
-Recognized by Tc cells
-MHC class II = Found only on antigen-presenting cells
-Recognized by TH cells
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T Cells
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T Cells
Cytotoxic T cells
-Naïve TC cells are activated upon TCR recognition of foreign peptide displayed on self-MHC class I protein on dendritic cells
-Clonal expansion and differentiation into activated cells and memory cells
-Activated cells induce apoptosis in cells with same specificity as first cell
-Likely a viral-infected or cancer cell
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T Cells
Helper T cells
-TH cells respond to exogenous antigen that is taken up by an antigen presenting cell
-Antigen is partially digested, then complexed with MHC class II proteins
-Complex is transported to and displayed on the cell surface
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T Cells
Helper T cells
-Activated TH cell gives rise to a clone of TH cells including both effector cells and memory cells
-Most effector TH cells leave the lymphoid organs and circulate around the body
-Secrete proteins called cytokines
-Promote humoral and cell-mediated immune responses
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B Cells
Humoral immunity begins when naïve B cells in secondary lymph organs meet antigens
-B cells are activated when their surface Igs bind to a specific epitope on an antigen
-TH cytokines may also be required
-Activation results in clonal expansion and differentiation into plasma and memory cells
-Plasma cells produce soluble antibodies against the same epitope
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Immunoglobulins
An immunoglobulin consists of two identical short polypeptides, light chains, and two identical longer polypeptides, heavy chains
-Four chains are held by disulfide bonds, forming a Y-shaped molecule
-Fab regions = Two “arms”
-Fc region = “Stem”
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Immunoglobulins
Each chain has a variable region (amino acid sequence differs between Igs) and a constant region-The variable regions fold together to form a cleft, the antigen-binding site
Each Ig can bind two identical epitopes-Allows formation of antigen-antibody complexes
-Indeed, Igs can agglutinate, precipitate or neutralize antigens
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Immunoglobulins
There are five classes of immunoglobulins
-IgM = First type of antibody produced during an immune response
-Monomer on B cells, but secreted as pentamer
-IgD = Present on mature naïve B cells
-Not secreted in normal situations
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Immunoglobulins
-IgG = Major form of antibody in blood
-Main component of secondary response
-Can cross placenta
-IgA = Major form of antibody in secretions
-Usually produced as a dimer
-Can pass to nursing infant in mom’s milk
-IgE = Present at very low levels in blood
-Plays a role in allergic reactions
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Immunoglobulin Diversity
Human B cells can generate antibodies with over 1010 different antigen-binding sites
-This diversity is generated through a process called DNA rearrangement
An Ig protein is encoded by different segments of DNA
-V (variable), D (diversity), J (joining)
-Plus a constant region
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Immunoglobulin Diversity
These segments are joined during maturation in the bone marrow
-First, a D and a J segment are joined
-Then, DJ is combined with a V segment
-Transcription and RNA processing follow, linking variable region to a constant region
-Translation occurs in the rough ER, where heavy and light chains are joined together
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T Cell Receptors
The structure of a TCR is similar to an immunoglobulin Fab
-Unlike Igs, TCRs are not secreted
-TCR diversity is also caused by DNA rearrangements
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Immune Responses
The first encounter with a foreign antigen is called the primary immune response
-Only few B or T cells can recognize antigen
The second encounter is called the secondary immune response
-This time there is a large clone of memory cells that can recognize the antigen
-Immune response is more effective
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Autoimmunity
The acceptance of self cells is known as immune tolerance
Autoimmune diseases are caused by the failure of immune tolerance
-Result in activation of autoreactive T cells, and production of autoantibodies by B cells
-Cause inflammation and organ damage
-Alleviated by corticosteroids and NSAIDs, including aspirin
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Allergy
Allergy refers to a greatly heightened response to a foreign antigen, or allergen
-The most common type is known as immediate hypersensitivity
-Results from excessive IgE production
-Seasonal hay fever
-Provoked by ragweed or other pollen
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Allergy
On initial exposure to allergen, B cells are activated to secrete IgE antibodies
-Bind to FC receptors on mast cells or basophils
On subsequent exposure to allergen, allergen cross-links bound IgEs
-Cells are induced to release histamine and other inflammatory mediators
-Produce symptoms of allergy
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Allergy
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Allergy (Cont.)
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Allergy
In systemic anaphylaxis, the allergic reaction is severe and potentially life-threatening
-Anaphylactic shock = Blood pressure drop, and bronchial constriction
-Death within 20-30 minutes
Most people, however, experience local anaphylaxis
-Hives or mild asthma
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Allergy
Delayed-type hypersensitivity produces symptoms within about 48 hours of a second exposure to an allergen
-Mediated by TH cells and macrophages
-Contact dermatitis
-Caused by varied materials, such as poison ivy, nickel in jewelry and
cosmetics
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Antibodies in Medicine
A person’s blood type is determined by antigens found on surface of red blood cells
-ABO blood types = Types A, B, AB and O
-Rh factor = Rh positive and Rh negative
The immune system is tolerant of its own RBC antigens, but makes antibodies that bind to those that differ
-For example, people with type A blood make antibodies against the B antigen
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Antibodies in Medicine
In blood transfusions, the antigens of the donor have to be matched to the antibodies of the recipient
-For instance, a type A person cannot donate to a type B or type O
-These would have anti-A antibodies
Blood is typed by agglutination reactions, using circulating IgM antibodies
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Antibodies in Medicine
A mismatched blood transfusion may be deadly
-Within 5-8 hours, tremendous hemolysis of the transfused RBCs is detected
-Due to formation of complement MACs
-The released hemoglobin is converted to bilirubin
-Can cause severe organ damage, especially to kidneys
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Monoclonal Antibodies
Monoclonal antibodies exhibit specificity for one epitope only
Animal is immunized with antigen then killed
-B cells are obtained from animal’s spleen
-Fused with a myeloma cell – a B-cell tumor that no longer produces Igs
-A clonal hybrid or hybridoma
-Divides indefinitely and produces monoclonal antibodies
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Monoclonal Antibodies
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Monoclonal Antibodies
Availability of large quantities of pure monoclonal antibodies has allowed the development of very sensitive clinical tests
-Some pregnancy tests use a monoclonal antibody against the hormone human chorionic gonadotropin (HCG)
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Monoclonal Antibodies
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Monoclonal Antibodies (Cont.)
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Monoclonal Antibodies
Monoclonal antibodies against CD4, a TH marker, are used to monitor AIDS
Monoclonal antibodies have also been used in the treatment of cancer
-Formation of tumor-specific immunotoxins
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Monoclonal Antibodies
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Monoclonal Antibodies (Cont.)
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Evading the Immune System
Some pathogens can alter their surface antigens to avoid immune system detection
Influenza virus expresses 2 surface proteins: hemaglutinin (HA) and neuraminidase (NA)
-Antigenic drift = Point mutations to the HA and NA genes
-Antigenic shift = Sudden appearance of a new virus subtype where HA and/or NA proteins are completely different
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Evading the Immune System
Salmonella typhimurium
-Can alternate between expression of two different flagellar proteins
Mycobacterium tuberculosis
-Once phagocytosed, inhibits fusion of the phagosome with lysosomes
Neisseria gonorrhoeae
-Secrete proteases that degrade IgA antibodies
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Evading the Immune System
HIV, human immunodeficiency virus, mounts a direct attack on TH cells
-Binds to CD4 proteins, and is endocytosed
An individual is considered to have AIDS when their TH cell level has dropped significantly
-Immunosuppression results in an increase in opportunistic infections and cancers
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Evading the Immune System