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The body’s defenders. Core concepts. Infectious diseases are caused by pathogens Nonspecific defenses against infection Plants and animals have mechanisms that are not targeted to specific pathogens that help them combat infection

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The body’s defenders

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The body s defenders

The body’s defenders

Core concepts

Core concepts

  • Infectious diseases are caused by pathogens

  • Nonspecific defenses against infection

    • Plants and animals have mechanisms that are not targeted to specific pathogens that help them combat infection

    • Skin and mucous membranes provide first-line barriers to infection

    • Phagocytic cells, inflammation, and antimicrobial proteins function as the second line of defense

  • Specific immunity arises from lymphocyte-antigen interactions

    • Lymphocytes provide the specificity and diversity of the immune system

    • Antigens interact with specific lymphocytes, inducing immune responses and immunological memory

    • Lymphocyte development gives rise to an immune system that distinguishes self from nonself

  • Immune responses take two forms: humoral and cell-mediated

    • Helper T-lymphocytes function in both humoral and cell-mediated immunity

    • Cytotoxic T-cells counter intracellular pathogens

    • B-cells make antibodies against extracellular pathogens

    • Memory B- and T-cells are responsible for faster and stronger secondary immune responses

  • Immunity in health and disease

    • Immunity can be achieved naturally or artificially

    • The immune system limits blood transfusion and tissue transplantation

    • Abnormal immune function can lead to disease

    • AIDS is an immunodeficiency disease caused by a virus



  • memory cell

  • monocytes

  • natural killer cells

  • neutrophils

  • nonspecific defense

  • opportunistic disease

  • passive immunity pathogen

  • perforin

  • phagocytosis

  • plasma cell

  • primary immune response

  • prostaglandins

  • pyrogens

  • Rh factor

  • secondary immune response

  • suppressor T cell

  • T cell

  • T cell receptor

  • target cell

  • tumor antigen

  • vaccine

  • chemokines

  • class I MHC

  • class II MHC

  • clonal selection

  • complement fixation

  • complement system

  • cytokine

  • cytotoxic T cell

  • effector cell

  • eosinophils

  • helper T cell

  • histamine

  • HIV

  • humoral immunity

  • immunity

  • immunodeficiency disease

  • immunoglobulin

  • inflammatory response

  • interferon

  • interleukin

  • lysozyme

  • macrophages

  • major histocompatibility complex

  • mast cells

  • membrane attack complex

  • ABO blood groups

  • active immunity

  • agglutination

  • AIDS

  • allergy

  • anaphylactic shock

  • antibody

  • antigen

  • antigen receptor

  • antigen-presenting cell

  • apoptosis

  • autoimmune disease

  • B cell

  • basophils

  • CD4

  • CD8

  • cell-mediated immunity

Pathogens and disease

Pathogens and disease





Large parasites

Two major types of defenses



Rapid responses to a

broad range of microbes


Slower responses to

specific microbes

External defenses

Internal defenses


Phagocytic cells

Humoral response


Mucous membranes

Antimicrobial proteins


Inflammatory response

Cell-mediated response






Natural killer cells

Two major types of defenses

Innate nonspecific immunity first line external defenses

Innate (nonspecific) immunity First line: External defenses

Mucus, cilia


Mouth bacteria, saliva

Skin, oil, sweat, acidity

Intestinal flora

Gastric juice

Acid conditions

Innate immunity second line internal defenses







Innate immunity Second line: Internal defenses

Phagocytic cell


  • Phagocytes

    • Attach to and ingest invading microorganisms

    • Initiate the inflammatory response

      • Macrophages – migrants or in lymph organs, lungs, kidneys, connective tissues

  • Antimicrobial proteins

    • Complement system – lysis of invading cells, triggers inflammation

    • Interferons – activate macrophages, prevent cell-to-cell spread of viruses

    • Defensins – secreted by macrophages to damage pathogens






The body s defenders






Innate immunity second line internal defenses con t

Innate immunity Second line: Internal defenses (con’t.)

  • Inflammatory response

  • Chemicals involved

  • Histamines

  • Prostaglandins

  • Chemokines

  • Pyrogens

Innate immunity second line internal defenses con t1

Innate immunity Second line: Internal defenses (con’t.)

  • Natural killer (NK) cells

    • Attack virus-infected body cells and cancer cells

    • Apoptosis (cell death) in cells attacked

  • Coelomocytes and hemocytes – phagocytes in invertebrates

Specific acquired immunity third line of defense

Specific (acquired) immunityThird line of defense


  • In blood and lymph

  • Types

    • B – cells – mature in marrow

    • T – cells – mature in thymus

      • Helper

      • Cytotoxic/Killer

      • Regulatory/Suppressor

      • Memory

  • Activated by cytokines from phagocytes

  • Display specificity to epitopes on antigens (antibody generator)

  • Have specific membrane-bound antigen-receptors

The body s defenders

  • Two types of specific immune responses

  • B and T cells generate clones of

  • short-lived activated effector cells

  • long-lived memory cells

Mhc molecules and t cell function

MHC molecules and T cell function

  • Class I MHC molecules

    • Most nucleated cells of the body

    • Infected/cancerous cells display parts of foreign antigens on surfaces

    • Recognized by cytotoxic T cells

  • Class II MHC molecules

    • Dendritic cells, macrophages, B cells (APCs) display phagocytized antigen fragments on surfaces

    • Recognized by helper T cells

  • T cells that have receptors for self-molecules are destroyed  self-tolerance

Clonal selection antigen driven cloning of lymphocytes



Clonal selectionantigen-driven cloning of lymphocytes

Antibody action

Antibody action

Immunity memory cells initiate a faster more efficient response upon reinfection

Immunity– memory cells initiate a faster, more efficient response upon reinfection

The body s defenders

Active immunity

  • Own system develops antibodies

  • Develops naturally in response to infection

  • Develops following immunization (artificial immunity)

  • Long-lasting protection but may take a long time

Passive immunity

  • Antibodies are passed from mother to fetus via the placenta

  • Antibodies are passed from mother to infant via breast milk (colostrum)

  • Antibodies may be injected into a nonimmune person (artificial immunity)

  • Immediate, short-term protection

Blood groups and transfusions

Blood groups and transfusions

  • Problems with transfusions and transplants

  • Antigens on RBC’s will determine a person’s blood type: A, B, AB, O blood

  • Another RBC antigen: Rh factor  Rh+ or Rh-

Immune disorders diseases

Immune disorders/diseases

  • Allergies – hypersensitive responses to antigens called allergens

  • Autoimmune diseases – immune system loses tolerance for self and turns against certain molecules of the body (SLE, diabetes type I, rheumatoid arthritis)

  • Immunodeficient diseases

    • Inborn or primary (severe combined ID)

    • Acquired or secondary

      • AIDS – HIV attacks CD4 molecules on helper T cells, macrophages, dendritic cells

The body s defenders








Mast cell


Degranulation of the cell,

triggered by cross-linking of

adjacent IgE molecules,

releases histamine and other

chemicals, leading to allergy




IgE antibodies bind to

receptors or mast cells.

On subsequent exposure to the

same allergen, IgE molecules

attached to a mast cell recog-

nize and bind the allergen.

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