The Immune System
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The Immune System. Overview. Barriers = 1 st line of defense against pathogens Immune system recognizes foreign bodies  responds with the production of leukocytes and proteins 2 kinds of defense: innate immunity acquired immunity. Pathogens (microorganisms and viruses).

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The Immune System

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The immune system

The Immune System


Overview

Overview

  • Barriers = 1st line of defense against pathogens

  • Immune system recognizes foreign bodies

     responds with the production of leukocytes and proteins

  • 2 kinds of defense:

    • innate immunity

    • acquired immunity


The immune system

Pathogens

(microorganisms

and viruses)

Barrier defenses:

Skin

Mucous membranes

Secretions

INNATE IMMUNITY

Recognition of traits

shared by broad ranges

of pathogens, using a

small set of receptors

Internal defenses:

Phagocytic cells

Antimicrobial proteins

Inflammatory response

Natural killer cells

Rapid response

Humoral response:

Antibodies defend against

infection in body fluids.

ACQUIRED IMMUNITY

Recognition of traits

specific to particular

pathogens, using a vast

array of receptors

Cell-mediated response:

Cytotoxic lymphocytes defend

against infection in body cells.

Slower response


The immune system

Innate immunity

  • present from birth

  • recognition and rapid response rely on shared traits of pathogens

  • nonspecific responses to pathogens

    • external barriers

    • internal cellular defense

    • chemical defenses

    • inflammation


Barriers

Barriers

  • Skin - continuous keratinized stratified squamous layers of epithelium

  • Mucus traps and allows for the removal of microbes

  • Saliva, mucus, and tears are hostile to microbes (hydrolytic enzymes, high salinity)

  • low pH of skin and digestive system prevents microbial growth


Cellular innate defenses

Cellular Innate Defenses

EXTRACELLULAR

FLUID

Lipopolysaccharide

Helper

protein

  • Phagocytic leukocytes engulf pathogens in the body

  • Groups of pathogens are recognized by TLR, Toll-like receptors of leukocytes

Flagellin

TLR4

WHITE

BLOOD

CELL

TLR5

VESICLE

TLR9

CpG DNA

Inflammatory

responses

TLR3

ds RNA


Phagocytic cells

Phagocytic Cells

4 types:

  • Neutrophils engulf and destroy microbes

  • Macrophages (of lymphatic system) and are found throughout the body

  • Eosinophils discharge destructive enzymes

  • Dendritic cells stimulate acquired immunity


Chemical defenses

Chemical Defenses

  • Proteins and enzymes attack microbes directly or impede their reproduction

  • Interferon proteins:

    • innate defense against viruses

    • activate macrophages

  • ~ 30 proteins form the complement system:

    lysis of invading cells, trigger inflammation


Inflammatory response

Inflammatory Response

  • Following an injury, mast cells release histamine:

    • Dilates blood vessels

    • increase local blood supply

    • more phagocytes and antimicrobial proteins enter tissues

  • Pus, a fluid rich in white blood cells, dead microbes, and cell debris, accumulates at the site of inflammation


Inflammatory response1

Inflammatory Response

  • Fever

    • 100 – 102 Fahrenheit = beneficial

    • Slows growth and reproduction of microbes

      • denature their proteins


The immune system

Pathogen

Splinter

Chemical

signals

Macrophage

Fluid

Mast cell

Capillary

Phagocytosis

Red blood cells

Phagocytic cell


Natural killer cells

Natural Killer Cells

  • All cells in the body (except red blood cells) have a class 1 MHC protein on their surface (major histocompatibility complex)

  • Cancerous or infected cells lack this protein; natural killer (NK) cells attack these damaged cells


Innate immune system evasion by pathogens

Innate Immune System Evasion by Pathogens

  • Some pathogens avoid destruction by modifying their surface to prevent recognition or by resisting breakdown following phagocytosis

  • Ex. Tuberculosis (TB);

    • kills more than a million people/year


The immune system

Acquired immunity

  • develops after exposure to foreign substances

  • specific responses

    • Antibody-mediated

    • Cell-mediated response

  • Slower


Lymphocyte dependent

Lymphocyte-Dependent

  • lymphocytes recognize, remember, and respond to antigens, foreign molecules

  • Cytokines, secreted by macrophages and dendritic cells, recruit and activate lymphocytes

  • T cells Lymphocytes mature in the thymus above the heart

  • B cells mature in bone marrow


B and t cell specificity

B and T-cell Specificity

  • Each lymphocyte is specialized to recognize ONE specific type of antigen (foreign body)

  • A single B cell or T cell has about 100,000 identical antigen receptors

  • B cells give rise to plasma cells,which secrete proteins called antibodies or immunoglobulins (Ig) specific to the antigen


The immune system

Fig. 43-9

Antigen-

binding

site

Antigen-

binding site

Antigen-

binding

site

Disulfide

bridge

V

V

V

V

Variable

regions

V

V

C

C

Constant

regions

C

C

C

C

Light

chain

Transmembrane

region

Plasma

membrane

 chain

 chain

Heavy chains

Disulfide bridge

B cell

Cytoplasm of B cell

Cytoplasm of T cell

T cell

(a) B cell receptor

(b) T cell receptor


The role of antibodies

The Role of Antibodies

  • Neutralization occurs when a pathogen can no longer infect a host because it is bound to an antibody

  • Opsonization occurs when antibodies bound to antigens increase phagocytosis

  • Antibodies together with proteins of the complement system generate a membrane attack complex and cell lysis

Animation: Antibodies


The immune system

Fig. 43-10

Antigen-

binding

sites

Epitopes

(antigenic

determinants)

Antigen-binding sites

Antigen

Antibody A

Antibody C

V

V

V

V

C

C

C

C

Antibody B


The immune system

Primary Immune Response

  • The first exposure to an antigen

  • Slow response

    • B cells called plasma cells are generated,

    • T cells are activated

  • In secondary immune response, B-cell memory facilitate a faster, more efficient response

Animation: Role of B Cells


Two branches of acquired immunity

Two branches of Acquired immunity

  • Humoral (antibody-mediated) immune response

    • activation and clonal selection of B cells, resulting in production of secreted antibodies

  • Cell-mediated immune response

    • activation and clonal selection of cytotoxic T cells

  • Helper T cells aid both responses


The immune system

Fig. 43-16

Humoral (antibody-mediated) immune response

Cell-mediated immune response

Key

Antigen (1st exposure)

Stimulates

Gives rise to

+

Engulfed by

Antigen-

presenting cell

+

+

+

B cell

Helper T cell

Cytotoxic T cell

+

+

Memory

Helper T cells

+

+

+

Antigen (2nd exposure)

Memory

Cytotoxic T cells

Active

Cytotoxic T cells

+

Plasma cells

Memory B cells

Secreted

antibodies

Defend against extracellular pathogens by binding to antigens,

thereby neutralizing pathogens or making them better targets

for phagocytes and complement proteins.

Defend against intracellular pathogens

and cancer by binding to and lysing the

infected cells or cancer cells.


Active immunity

Active Immunity

  • Active immunity develops naturally in response to an infection

  • It can also develop following vaccination

    • a nonpathogenic form of a microbe or part of a microbe is used to initiate a primary immune response and immunological memory


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