1

2

Pathogens(such as bacteria,fungi, and viruses)

INNATE IMMUNITY(all animals)

• Rapid response

Recognition of traits sharedby broad ranges ofpathogens, using a smallset of receptors

•

Recognition of traits specific to particularpathogens, using a vastarray of receptors

•

• Slower response

Barrier defenses:SkinMucous membranesSecretions

Internal defenses:Phagocytic cellsNatural killer cellsAntimicrobial proteinsInflammatory response

Humoral response:Antibodies defend againstinfection in body fluids.

Cell-mediated response:Cytotoxic cells defendagainst infection in body cells.

ADAPTIVE IMMUNITY(vertebrates only)

3

Pathogen

PHAGOCYTICCELL

VacuoleLysosomecontainingenzymes

4

Thymus

Peyer’spatches(smallintestine)

Appendix(cecum)

AdenoidTonsils

Lymphaticvessels

Spleen

Lymphnodes

Lymphnode

Bloodcapillary

Interstitialfluid

Tissuecells

Lymphatic vessel

Lymphatic vessel

Masses ofdefensive cells

5

Pathogen Splinter

Mastcell

Macro-phage

Capillary

Redblood cells

Neutrophil

Signalingmolecules

Movementof fluid

Phagocytosis

6

Antigen receptors

Mature B cell Mature T cell

7Cytoplasm of B cell

Antigen-binding site

B cellantigenreceptor

B cell

Lightchain

Disulfidebridge

Antigen-binding site

Variable regions

Constant regions

Transmembraneregion

Heavy chains

Plasmamembrane

C C

C C

V

V

V

V

8

AntibodyAntigenreceptor

B cell

Antigen Epitope

Pathogen(a) B cell antigen receptors and antibodies

Antibody C

Antibody BAntibody A

Antigen

(b) Antigen receptor specificity

9

T cellantigenreceptor

T cell Cytoplasm of T cell

Plasmamembrane

chain chain

Disulfidebridge

Antigen-bindingsite

Variableregions

Constantregions

Transmembraneregion

V V

C C

10

Displayedantigenfragment

MHC molecule

Antigenfragment

Pathogen

Host cell

T cell

T cell antigenreceptor

(a) Antigen recognition by a T cell

(b) A closer look at antigen presentation

Antigen fragment

MHCmolecule

Host cell

Top view

11

DNA ofundifferentiatedB cell

DNA ofdifferentiatedB cell

Recombination deletes DNA betweenrandomly selected V segment and J segment

Functional gene

Transcription

RNA processing

Translation

pre-mRNA

mRNA

Light-chain polypeptideAntigen receptor

B cellVariableregion

Constantregion

2

3

4

1

Poly-A tailCap V39 J5

J5V39

V37 V38 V39

V37 V38 V39 V40

J5

J5J4J3J2J1

Intron

Intron

V C

C

C

C

C

C

C C

C

VVV

V

Intron

12

AntigenAntigenreceptor

Antibody

Plasma cellsMemory cells

B cells thatdiffer inantigenspecificity

13

Antigen-presentingcell

Pathogen

Antigen fragment

Class II MHC moleculeAccessory proteinAntigen receptor

Helper T cell

Cytokines

Humoralimmunity

Cell-mediatedimmunity

B cellCytotoxic T cell

3

2

1

14

Cytotoxic T cell

31 2

Accessoryprotein

Class I MHCmolecule

Infectedcell

Antigenreceptor

Antigenfragment

Perforin

Pore

Gran-zymes

ReleasedcytotoxicT cell

Dyinginfected cell

15

Pathogen

31 2

Antigen-presentingcell Antigen

fragment

Class IIMHC

molecule

Antigenreceptor

Accessoryprotein

Helper T cell

B cell

Cytokines

Activatedhelper T cell

Memory B cells

Plasma cellsSecreted

antibodies

16

OpsonizationNeutralization

Antibody

VirusBacterium

Macrophage

Activation of complement system and poreformation

Complement proteins

Formation of membraneattack complex

Flow of waterand ions

Pore

AntigenForeigncell

17

IgE

Allergen

Histamine

Granule

Mast cell