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Chapter 43 Immune System
Innate Immunity
• This type of immunity is present before any exposure to pathogens and begins at birth
• Largely nonspecific• Skin is the first line of defense• Mucous membranes that line the digestive, respiratory,
and genitourinary tracts• Tears, sweat, and saliva are body secretions that serve
this type of immunity• Lysozyme is an antimicrobial protein found in the skin
and mucous membrane that digests cell walls of bacteria
Innate Immunity• If microbes make it past the external defenses then cellular
and chemical defenses take over• White blood cells that serve as phagocytes that engulf and
destroy microbes- also initiate the inflammatory response
Four types of phagocytic WBC:1. Neutrophils – most abundant; life span a few days2. Macrophages – larger, longer living WBC, but only 5% in circulation; found in spleen, lymph nodes3. Eosinophils – low activity, but essential to defend against multicellular parasites4. Dendritic cells – ingest much like macrophages
Phagocytosis
Innate Immunity• Antimicrobial proteins directly attack proteins or impede
their reproduction- complement system – 30 serum proteins that are inactive without microbes-trigger cascade of steps that lyse invading cells; also trigger inflammationInterferon (alpha & beta) – proteins that defend against viral infections- secreted by virus infected cells to induce other cells to produce substances to inhibit further infection
Innate Immunity• Inflammatory response – triggered from injury or entry of
pathogens to a site of body- Mast cells found in connective tissue release histamine- release will trigger dilation and permeability of
capillaries- increased blood flow causes redness and heat- vascular changes deliver antimicrobial proteins and
clotting elements to site which begins repair and blocks spread to other parts of body
- Chemokines – small proteins that direct the migration of phagocytes and increase production of microbe killing compounds
Inflammatory Reponse
Innate Immunity
• Fever is another systemic response in which toxins from pathogen and substances from macrophages set body’s temperature higher
• Natural Killer cells – nonspecific – patrol and attack virus infected or cancer cells
- surface receptors recognize the target and release chemicals that cause
apoptosis or programmed cell death
Summary
Acquired Immunity
• Develops only after exposure to inducing agents and is highly specific
• Cytokines proteins secreted from macrophages that activate the specific WBC called lymphocytes
• Antigen – any foreign molecule recognized by lymphocytes and elicits a response
epitope – binding or recognition site on the antigen that elicits a response• Antibodies – defensive proteins released by lymphocytes
that recognize the epitope of antigens and target them for destruction
Acquired ImmunityTwo types of lymphocytes:
1. B Lymphocytes2. T Lymphocytes
- Both use antigen specific receptors to recognize antigens- B cells recognize intact antigens, whereas T cells can
recognize fragments of antigens
Acquired Immunity• Major Histocompatibility Complex (MHC) – family
of genes that normal cell surface proteins- these proteins attach to antigens and present them to the surface of the plasma membrane for recognition by a particular T cell- Class I MHC – found on almost all nucleated cells of the body and recognized Cytotoxic T cells- Class II MHC – made by dendritic, macrophages and B cells and recognized by Helper T cells
Lymphocyte Development• Both lymphocytes develop from pluripotent stem cells
- T cells – migrate to thymus, gland of thoracic cavity, during maturation
- B cells – stay in bone marrow during maturation (bursa of Fabricius)Three key events of lymphocytes
1. maturation of B cell in marrow2. maturation of T cell in thymus3. Lymphocyte encounters and binds to antigens
leading to activation, proliferation, and differentiation – clonal selection
Clonal Selection
• Clonal selection – antigen driven cloning of lymphocytes that selectively activates tiny fraction of cells that are specific for and dedicated to eliminating that antigen
- effector cells – clone of large numbered short lived cells that combat antigen
- memory cells – long lived cells bearing receptors specific for same inducing antigen
Clonal Selection
• Primary immune response – proliferation and differentiation of lymphocytes the first time the body is exposed to antigen
- response peak in 10-17 days• Secondary immune response – exposure to
the same antigen that produces a faster, more prolonged response of greater magnitude
Acquired Immunity
• Two branches of immunity:1. Humoral response – involves
activation and clonal selection of B cells that release antibodies into blood and lymph
2. Cell-mediated response – activation and clonal selection of cytotoxic T cells that
directly destroy target cells
Acquired Immunity
• Central to humoral and cell mediated responses is the helper T cell- CD4 is a surface protein on the helper T cell that
binds the class II MHC- keeps helper T and antigen presenting cell joined during activation of helper T clones
and memory cells- leads to secretion of cytokines that
stimulate other lymphocytes
Acquired Immunity
• Cytotoxic T cells target virus infected cells and cancer cells
- utilize CD8 surface protein that behaves like CD4 in helper T cells- Activated Cytotoxic T cells secrete
proteins that act on bound infected cell and destroy it
- exposes pathogen to antibodies
Antibody Classes• 5 classes - binding sites are
responsible for identification of antigens- tails are responsible for antibody distribution and mediation of antigen disposal
Antibody Disposal• 5 mechanisms of disposal by antibodies
1. Viral neutralization – binding of proteins to surface blocks ability to infect host
2. Opsonization – bound antibodies enhance macrophage attachment to microbes and increase phagocytosis
3. Agglutination – clumping of bacteria or viruses forms aggregates that can be readily phagocytosed by macrophages
4. Precipitation – antibodies cross link soluble antigen molecules dissolved in body fluids making them immobile and easily targeted by phagocytes
5. Complement system – binding of antigen-antibody complex causes complement proteins to activate a cascade of reactions that generate a Membrane Attack complex (MAC)*forms a pore in membrane, ions and water rush in and
cause cell to swell and lyse
Active & Passive Immunity
• Active immunity – occurs by natural exposure to infectious agent -can develop from immunizations or vaccinations with inactivated form of bacterial toxin, microbe, parts of microbe, genes of microbial proteins
• Passive Immunity – conferred by transferring antibodies from one immune individual to one that has not been exposed to agentEx: mother passing antibodies to fetus through colostrum
Blood Groups and Transfusions• Blood types contain antigens that the body
recognizes as self and non-selfEx: Type A blood with A antigens is
tolerant, but Type B blood will generate antibodies and lead to a transfusion reaction that can involves lyses of red blood cells and can lead to chills, fever, shock, and kidney malfunction
Blood Groups
• Rh factor – red blood cell antigen that triggers antibodies (IgG) to mount a humoral response on fetus
- Mothers can be injected with anti-Rh IgG antibodies to prevent attack on future fetuses
Allergies
• Allergies – exaggerated or hypersensitive responses to a certain antigen or allergen
• Most common allergies involve IgE antibodies ( hay fever)induction of mast cells cause release of histamine
that dialates blood vessels- symptoms runny nose, sneezing, tearing eyes
Antihistamines block histamine receptors and diminish allergy symptoms
Anaphylactic shock – whole body, life threatening reaction with exposure to antigen
Ex: peanut allergies, bee stings
Autoimmune Disease
• Immune system loses tolerance of self and turns against certain molecules of body
• - releases antibodies against self molecules like histones and DNA
• - thought to arise from some failure of immune system regulation