Immunity: Two Intrinsic Defense Systems

Immunity: Two Intrinsic Defense Immunity: Two Intrinsic Defense SystemsSystems

Innate (nonspecific) system responds quickly Innate (nonspecific) system responds quickly and consists of:and consists of: First line of defense – skin and mucosae prevent First line of defense – skin and mucosae prevent

entry of microorganismsentry of microorganisms Second line of defense – antimicrobial proteins, Second line of defense – antimicrobial proteins,

phagocytes, and other cells phagocytes, and other cells Inhibit spread of invaders throughout the bodyInhibit spread of invaders throughout the body Inflammation is its most important mechanism Inflammation is its most important mechanism

Immunity: Two Intrinsic Defense Immunity: Two Intrinsic Defense SystemsSystems

Adaptive (specific) defense systemAdaptive (specific) defense system Third line of defense – mounts attack against Third line of defense – mounts attack against

particular foreign substancesparticular foreign substances Takes longer to react than the innate systemTakes longer to react than the innate system Works in conjunction with the innate systemWorks in conjunction with the innate system

Innate and Adaptive DefensesInnate and Adaptive Defenses

Figure 21.1

Surface BarriersSurface Barriers Skin, mucous membranes, and their secretions Skin, mucous membranes, and their secretions

make up the first line of defensemake up the first line of defense Keratin in the skin:Keratin in the skin:

Presents a physical barrier to most microorganismsPresents a physical barrier to most microorganisms Is resistant to weak acids and bases, bacterial Is resistant to weak acids and bases, bacterial

enzymes, and toxinsenzymes, and toxins Mucosae provide similar mechanical barriersMucosae provide similar mechanical barriers

Epithelial Chemical BarriersEpithelial Chemical Barriers Epithelial membranes produce protective Epithelial membranes produce protective

chemicals that destroy microorganismschemicals that destroy microorganisms Skin acidity (pH of 3 to 5) inhibits bacterial Skin acidity (pH of 3 to 5) inhibits bacterial

growthgrowth Sebum contains chemicals toxic to bacteriaSebum contains chemicals toxic to bacteria Stomach mucosae secrete concentrated HCl and Stomach mucosae secrete concentrated HCl and

protein-digesting enzymesprotein-digesting enzymes Saliva and lacrimal fluid contain lysozymeSaliva and lacrimal fluid contain lysozyme Mucus traps microorganisms that enter the Mucus traps microorganisms that enter the

digestive and respiratory systemsdigestive and respiratory systems

Respiratory Tract MucosaeRespiratory Tract Mucosae Mucus-coated hairs in the nose trap inhaled Mucus-coated hairs in the nose trap inhaled

particlesparticles Mucosa of the upper respiratory tract is Mucosa of the upper respiratory tract is

ciliatedciliated Cilia sweep dust- and bacteria-laden mucus away Cilia sweep dust- and bacteria-laden mucus away

from lower respiratory passagesfrom lower respiratory passages

Internal Defenses: Cells and Internal Defenses: Cells and ChemicalsChemicals

The body uses nonspecific cellular and The body uses nonspecific cellular and chemical devices to protect itselfchemical devices to protect itself Phagocytes and natural killer (NK) cellsPhagocytes and natural killer (NK) cells Antimicrobial proteins in blood and tissue fluidAntimicrobial proteins in blood and tissue fluid Inflammatory response enlists macrophages, mast Inflammatory response enlists macrophages, mast

cells, WBCs, and chemicalscells, WBCs, and chemicals Harmful substances are identified by surface Harmful substances are identified by surface

carbohydrates unique to infectious organismscarbohydrates unique to infectious organisms

Figure 21.2a

PhagocytesPhagocytes Macrophages are the chief phagocytic cellsMacrophages are the chief phagocytic cells Free macrophages wander throughout a region Free macrophages wander throughout a region

in search of cellular debrisin search of cellular debris Kupffer cells (liver) and microglia (brain) are Kupffer cells (liver) and microglia (brain) are

fixed macrophagesfixed macrophages

PhagocytesPhagocytes Neutrophils become phagocytic when Neutrophils become phagocytic when

encountering infectious materialencountering infectious material Eosinophils are weakly phagocytic against Eosinophils are weakly phagocytic against

parasitic wormsparasitic worms Mast cells bind and ingest a wide range of Mast cells bind and ingest a wide range of

bacteriabacteria

Mechanism of PhagocytosisMechanism of Phagocytosis Microbes adhere to the phagocyteMicrobes adhere to the phagocyte Pseudopods engulf the particle (antigen) into a Pseudopods engulf the particle (antigen) into a

phagosomephagosome Phagosomes fuse with a lysosome to form a Phagosomes fuse with a lysosome to form a

phagolysosomephagolysosome Invaders in the phagolysosome are digested by Invaders in the phagolysosome are digested by

proteolytic enzymesproteolytic enzymes Indigestible and residual material is removed Indigestible and residual material is removed

by exocytosisby exocytosis

Natural Killer (NK) CellsNatural Killer (NK) Cells Can lyse and kill cancer cells and virus-Can lyse and kill cancer cells and virus-

infected cellsinfected cells Are a small, distinct group of large granular Are a small, distinct group of large granular

lymphocytes lymphocytes React nonspecifically and eliminate cancerous React nonspecifically and eliminate cancerous

and virus-infected cellsand virus-infected cells Kill their target cells by releasing perforins and Kill their target cells by releasing perforins and

other cytolytic chemicalsother cytolytic chemicals Secrete potent chemicals that enhance the Secrete potent chemicals that enhance the

inflammatory responseinflammatory response

Inflammation: Tissue Response Inflammation: Tissue Response to Injuryto Injury

The inflammatory response is triggered The inflammatory response is triggered whenever body tissues are injured whenever body tissues are injured Prevents the spread of damaging agents to nearby Prevents the spread of damaging agents to nearby

tissuestissues Disposes of cell debris and pathogensDisposes of cell debris and pathogens Sets the stage for repair processesSets the stage for repair processes

The four cardinal signs of acute inflammation The four cardinal signs of acute inflammation are redness, heat, swelling, and painare redness, heat, swelling, and pain

Inflammation ResponseInflammation Response Begins with a flood of inflammatory chemicals Begins with a flood of inflammatory chemicals

released into the extracellular fluidreleased into the extracellular fluid Inflammatory mediators:Inflammatory mediators:

Kinins, prostaglandins (PGs), complement, and Kinins, prostaglandins (PGs), complement, and cytokines cytokines

Released by injured tissue, phagocytes, Released by injured tissue, phagocytes, lymphocytes, and mast cellslymphocytes, and mast cells

Cause local small blood vessels to dilate, resulting Cause local small blood vessels to dilate, resulting in hyperemia in hyperemia

Toll-like Receptors (TLRs)Toll-like Receptors (TLRs) Macrophages and cells lining the Macrophages and cells lining the

gastrointestinal and respiratory tracts bear gastrointestinal and respiratory tracts bear TLRsTLRs

TLRs recognize specific classes of infecting TLRs recognize specific classes of infecting microbesmicrobes

Activated TLRs trigger the release of Activated TLRs trigger the release of cytokines that promote inflammationcytokines that promote inflammation

Inflammatory Response: Inflammatory Response: Vascular PermeabilityVascular Permeability

Chemicals liberated by the inflammatory Chemicals liberated by the inflammatory response increase the permeability of local response increase the permeability of local capillariescapillaries

Exudate—fluid containing proteins, clotting Exudate—fluid containing proteins, clotting factors, and antibodies factors, and antibodies Exudate seeps into tissue spaces causing local Exudate seeps into tissue spaces causing local

edema (swelling), which contributes to the edema (swelling), which contributes to the sensation of painsensation of pain

Inflammatory Response: EdemaInflammatory Response: Edema The surge of protein-rich fluids into tissue The surge of protein-rich fluids into tissue

spaces (edema):spaces (edema): Helps dilute harmful substancesHelps dilute harmful substances Brings in large quantities of oxygen and nutrients Brings in large quantities of oxygen and nutrients

needed for repairneeded for repair Allows entry of clotting proteins, which prevents Allows entry of clotting proteins, which prevents

the spread of bacteriathe spread of bacteria

Inflammatory Response: Inflammatory Response: Phagocytic MobilizationPhagocytic Mobilization

Four main phases:Four main phases: Leukocytosis – neutrophils are released from the Leukocytosis – neutrophils are released from the

bone marrow in response to leukocytosis-inducing bone marrow in response to leukocytosis-inducing factors released by injured cellsfactors released by injured cells

Margination – neutrophils cling to the walls of Margination – neutrophils cling to the walls of capillaries in the injured areacapillaries in the injured area

Diapedesis – neutrophils squeeze through capillary Diapedesis – neutrophils squeeze through capillary walls and begin phagocytosiswalls and begin phagocytosis

Chemotaxis – inflammatory chemicals attract Chemotaxis – inflammatory chemicals attract neutrophils to the injury siteneutrophils to the injury site

Antimicrobial ProteinsAntimicrobial Proteins Enhance the innate defenses by:Enhance the innate defenses by:

Attacking microorganisms directlyAttacking microorganisms directly Hindering microorganisms’ ability to reproduceHindering microorganisms’ ability to reproduce

The most important antimicrobial proteins are:The most important antimicrobial proteins are: InterferonInterferon Complement proteinsComplement proteins

Interferon (IFN)Interferon (IFN) Genes that synthesize IFN are activated when Genes that synthesize IFN are activated when

a host cell is invaded by a virusa host cell is invaded by a virus Interferon molecules leave the infected cell Interferon molecules leave the infected cell

and enter neighboring cellsand enter neighboring cells Interferon stimulates the neighboring cells to Interferon stimulates the neighboring cells to

activate genes for PKR (an antiviral protein)activate genes for PKR (an antiviral protein) PKR nonspecifically blocks viral reproduction PKR nonspecifically blocks viral reproduction

in the neighboring cellin the neighboring cell

Interferon FamilyInterferon Family Family of related proteins each with slightly Family of related proteins each with slightly

different physiological effectsdifferent physiological effects Lymphocytes secrete gamma (Lymphocytes secrete gamma () interferon, but ) interferon, but

most other WBCs secrete alpha (most other WBCs secrete alpha () interferon) interferon Fibroblasts secrete beta (Fibroblasts secrete beta () interferon) interferon Interferons also activate macrophages and Interferons also activate macrophages and

mobilize NKs mobilize NKs FDA-approved alpha IFN is used: FDA-approved alpha IFN is used:

As an antiviral drug against hepatitis C virusAs an antiviral drug against hepatitis C virus To treat genital warts caused by the herpes virus To treat genital warts caused by the herpes virus

ComplementComplement 20 or so proteins that circulate in the blood in 20 or so proteins that circulate in the blood in

an inactive forman inactive form Proteins include C1 through C9, factors B, D, Proteins include C1 through C9, factors B, D,

and P, and regulatory proteinsand P, and regulatory proteins Provides a major mechanism for destroying Provides a major mechanism for destroying

foreign substances in the bodyforeign substances in the body

ComplementComplement Amplifies all aspects of the inflammatory Amplifies all aspects of the inflammatory

responseresponse Kills bacteria and certain other cell types (our Kills bacteria and certain other cell types (our

cells are immune to complement)cells are immune to complement) Enhances the effectiveness of both nonspecific Enhances the effectiveness of both nonspecific

and specific defensesand specific defenses

Complement PathwaysComplement Pathways Complement can be activated by two Complement can be activated by two

pathways: classical and alternativepathways: classical and alternative Classical pathway is linked to the immune Classical pathway is linked to the immune

system system Depends on the binding of antibodies to invading Depends on the binding of antibodies to invading

organisms organisms Subsequent binding of C1 to the antigen-antibody Subsequent binding of C1 to the antigen-antibody

complexes (complement fixation)complexes (complement fixation) Alternative pathway is triggered by interaction Alternative pathway is triggered by interaction

among factors B, D, and P, and polysaccharide among factors B, D, and P, and polysaccharide molecules present on microorganismsmolecules present on microorganisms

Complement PathwaysComplement Pathways Each pathway involves a cascade in which Each pathway involves a cascade in which

complement proteins are activated in a complement proteins are activated in a sequence where each step catalyzes the nextsequence where each step catalyzes the next

Both pathways converge on C3, which cleaves Both pathways converge on C3, which cleaves into C3a and C3binto C3a and C3b

Complement PathwaysComplement Pathways C3b initiates formation of a membrane attack C3b initiates formation of a membrane attack

complex (MAC)complex (MAC) MAC causes cell lysis by interfering with a MAC causes cell lysis by interfering with a

cell’s ability to eject Cacell’s ability to eject Ca2+2+

C3b also causes opsonization, and C3a causes C3b also causes opsonization, and C3a causes inflammationinflammation

C-reactive Protein (CRP)C-reactive Protein (CRP) CRP is produced by the liver in response to CRP is produced by the liver in response to

inflammatory moleculesinflammatory molecules CRP is a clinical marker used to assess:CRP is a clinical marker used to assess:

The presence of an acute infectionThe presence of an acute infection An inflammatory condition and its response to An inflammatory condition and its response to

treatmenttreatment

Functions of C-reactive ProteinFunctions of C-reactive Protein Binds to PC receptor of pathogens and Binds to PC receptor of pathogens and

exposed self-antigensexposed self-antigens Plays a surveillance role in targeting damaged Plays a surveillance role in targeting damaged

cells for disposalcells for disposal Activates complementActivates complement

FeverFever Abnormally high body temperature in Abnormally high body temperature in

response to invading microorganismsresponse to invading microorganisms The body’s thermostat is reset upwards in The body’s thermostat is reset upwards in

response to pyrogens, chemicals secreted by response to pyrogens, chemicals secreted by leukocytes and macrophages exposed to leukocytes and macrophages exposed to bacteria and other foreign substancesbacteria and other foreign substances

FeverFever High fevers are dangerous because they can High fevers are dangerous because they can

denature enzymesdenature enzymes Moderate fever can be beneficial, as it causes:Moderate fever can be beneficial, as it causes:

The liver and spleen to sequester iron and zinc The liver and spleen to sequester iron and zinc (needed by microorganisms)(needed by microorganisms)

An increase in the metabolic rate, which speeds up An increase in the metabolic rate, which speeds up tissue repairtissue repair

Adaptive (Specific) DefensesAdaptive (Specific) Defenses The adaptive immune system is a functional The adaptive immune system is a functional

system that:system that: Recognizes specific foreign substancesRecognizes specific foreign substances Acts to immobilize, neutralize, or destroy foreign Acts to immobilize, neutralize, or destroy foreign

substancessubstances Amplifies inflammatory response and activates Amplifies inflammatory response and activates

complementcomplement

Adaptive Immune DefensesAdaptive Immune Defenses The adaptive immune system is antigen-The adaptive immune system is antigen-

specific, systemic, and has memoryspecific, systemic, and has memory It has two separate but overlapping arms:It has two separate but overlapping arms:

Humoral, or antibody-mediated immunityHumoral, or antibody-mediated immunity Cellular, or cell-mediated immunity Cellular, or cell-mediated immunity

AntigensAntigens Substances that can mobilize the immune Substances that can mobilize the immune

system and provoke an immune responsesystem and provoke an immune response The ultimate targets of all immune responses The ultimate targets of all immune responses

are mostly large, complex molecules not are mostly large, complex molecules not normally found in the body (nonself)normally found in the body (nonself)

Complete AntigensComplete Antigens Important functional properties:Important functional properties:

Immunogenicity – ability to stimulate proliferation Immunogenicity – ability to stimulate proliferation of specific lymphocytes and antibody productionof specific lymphocytes and antibody production

Reactivity – ability to react with products of Reactivity – ability to react with products of activated lymphocytes and the antibodies released activated lymphocytes and the antibodies released in response to themin response to them

Complete antigens include foreign protein, Complete antigens include foreign protein, nucleic acid, some lipids, and large nucleic acid, some lipids, and large polysaccharidespolysaccharides

Haptens (Incomplete Antigens)Haptens (Incomplete Antigens) Small molecules, such as peptides, nucleotides, Small molecules, such as peptides, nucleotides,

and many hormones, that are not immunogenic but and many hormones, that are not immunogenic but are reactive when attached to protein carriersare reactive when attached to protein carriers

If they link up with the body’s proteins, the If they link up with the body’s proteins, the adaptive immune system may recognize them as adaptive immune system may recognize them as foreign and mount a harmful attack (allergy)foreign and mount a harmful attack (allergy)

Haptens are found in poison ivy, dander, some Haptens are found in poison ivy, dander, some detergents, and cosmeticsdetergents, and cosmetics

Antigenic DeterminantsAntigenic Determinants Only certain parts of an entire antigen are Only certain parts of an entire antigen are

immunogenicimmunogenic Antibodies and activated lymphocytes bind to Antibodies and activated lymphocytes bind to

these antigenic determinantsthese antigenic determinants Most naturally occurring antigens have Most naturally occurring antigens have

numerous antigenic determinants that:numerous antigenic determinants that: Mobilize several different lymphocyte populationsMobilize several different lymphocyte populations Form different kinds of antibodies against itForm different kinds of antibodies against it

Large, chemically simple molecules (e.g., Large, chemically simple molecules (e.g., plastics) have little or no immunogenicityplastics) have little or no immunogenicity

Self-Antigens: MHC ProteinsSelf-Antigens: MHC Proteins Our cells are dotted with protein molecules Our cells are dotted with protein molecules

(self-antigens) that are not antigenic to us but (self-antigens) that are not antigenic to us but are strongly antigenic to others are strongly antigenic to others

One type, MHC proteins, mark a cell as selfOne type, MHC proteins, mark a cell as self The two classes of MHC proteins are:The two classes of MHC proteins are:

Class I MHC proteins – found on virtually all body Class I MHC proteins – found on virtually all body cellscells

Class II MHC proteins – found on certain cells in Class II MHC proteins – found on certain cells in the immune response the immune response

MHC ProteinsMHC Proteins Are coded for by genes of the major Are coded for by genes of the major

histocompatibility complex (MHC) and are histocompatibility complex (MHC) and are unique to an individualunique to an individual

Each MHC molecule has a deep groove that Each MHC molecule has a deep groove that displays a peptide, which is a normal cellular displays a peptide, which is a normal cellular product of protein recycling product of protein recycling

In infected cells, MHC proteins bind to In infected cells, MHC proteins bind to fragments of foreign antigens, which play a fragments of foreign antigens, which play a crucial role in mobilizing the immune systemcrucial role in mobilizing the immune system

Cells of the Adaptive Immune Cells of the Adaptive Immune SystemSystem

Two types of lymphocytesTwo types of lymphocytes B lymphocytes – oversee humoral immunityB lymphocytes – oversee humoral immunity T lymphocytes – non-antibody-producing cells that T lymphocytes – non-antibody-producing cells that

constitute the cell-mediated arm of immunityconstitute the cell-mediated arm of immunity Antigen-presenting cells (APCs):Antigen-presenting cells (APCs):

Do not respond to specific antigensDo not respond to specific antigens Play essential auxiliary roles in immunityPlay essential auxiliary roles in immunity

LymphocytesLymphocytes Immature lymphocytes released from bone Immature lymphocytes released from bone

marrow are essentially identicalmarrow are essentially identical Whether a lymphocyte matures into a B cell or Whether a lymphocyte matures into a B cell or

a T cell depends on where in the body it a T cell depends on where in the body it becomes immunocompetentbecomes immunocompetent B cells mature in the bone marrowB cells mature in the bone marrow T cells mature in the thymusT cells mature in the thymus

T CellsT Cells T cells mature in the thymus under negative T cells mature in the thymus under negative

and positive selection pressuresand positive selection pressures Negative selection – eliminates T cells that are Negative selection – eliminates T cells that are

strongly anti-selfstrongly anti-self Positive selection – selects T cells with a weak Positive selection – selects T cells with a weak

response to self-antigens, which thus become both response to self-antigens, which thus become both immunocompetent and self-tolerantimmunocompetent and self-tolerant

B CellsB Cells B cells become immunocompetent and self-B cells become immunocompetent and self-

tolerant in bone marrowtolerant in bone marrow Some self-reactive B cells are inactivated Some self-reactive B cells are inactivated

(anergy) while others are killed(anergy) while others are killed Other B cells undergo receptor editing in Other B cells undergo receptor editing in

which there is a rearrangement of their which there is a rearrangement of their receptorsreceptors

Immunocompetent B or T cellsImmunocompetent B or T cells Display a unique type of receptor that responds Display a unique type of receptor that responds

to a distinct antigento a distinct antigen Become immunocompetent before they Become immunocompetent before they

encounter antigens they may later attackencounter antigens they may later attack Are exported to secondary lymphoid tissue Are exported to secondary lymphoid tissue

where encounters with antigens occurwhere encounters with antigens occur Mature into fully functional antigen-activated Mature into fully functional antigen-activated

cells upon binding with their recognized cells upon binding with their recognized antigen antigen

It is genes, not antigens, that determine which It is genes, not antigens, that determine which foreign substances our immune system will foreign substances our immune system will recognize and resistrecognize and resist

Antigen-Presenting Cells (APCs)Antigen-Presenting Cells (APCs) Major rolls in immunity are:Major rolls in immunity are:

To engulf foreign particlesTo engulf foreign particles To present fragments of antigens on their own To present fragments of antigens on their own

surfaces, to be recognized by T cellssurfaces, to be recognized by T cells Major APCs are dendritic cells (DCs), Major APCs are dendritic cells (DCs),

macrophages, and activated B cellsmacrophages, and activated B cells The major initiators of adaptive immunity are The major initiators of adaptive immunity are

DCs, which migrate to the lymph nodes and DCs, which migrate to the lymph nodes and secondary lymphoid organs, and present secondary lymphoid organs, and present antigens to T and B cellsantigens to T and B cells

Macrophages and Dendritic CellsMacrophages and Dendritic Cells Secrete soluble proteins that activate T cellsSecrete soluble proteins that activate T cells Activated T cells in turn release chemicals Activated T cells in turn release chemicals

that:that: Rev up the maturation and mobilization of DCsRev up the maturation and mobilization of DCs Prod macrophages to become activated Prod macrophages to become activated

macrophages, which are insatiable phagocytes that macrophages, which are insatiable phagocytes that secrete bactericidal chemicalssecrete bactericidal chemicals

Adaptive Immunity: SummaryAdaptive Immunity: Summary Two-fisted defensive system that uses Two-fisted defensive system that uses

lymphocytes, APCs, and specific molecules to lymphocytes, APCs, and specific molecules to identify and destroy nonself particlesidentify and destroy nonself particles

Its response depends upon the ability of its cells Its response depends upon the ability of its cells to:to: Recognize foreign substances (antigens) by binding Recognize foreign substances (antigens) by binding

to themto them Communicate with one another so that the whole Communicate with one another so that the whole

system mounts a response specific to those antigenssystem mounts a response specific to those antigens

Humoral Immunity ResponseHumoral Immunity Response Antigen challenge – first encounter between an Antigen challenge – first encounter between an

antigen and a naive immunocompetent cellantigen and a naive immunocompetent cell Takes place in the spleen or other lymphoid Takes place in the spleen or other lymphoid

organorgan If the lymphocyte is a B cell:If the lymphocyte is a B cell:

The challenging antigen provokes a humoral The challenging antigen provokes a humoral immune responseimmune response

Antibodies are produced against the challenger Antibodies are produced against the challenger

Clonal SelectionClonal Selection Stimulated B cell growth forms clones bearing Stimulated B cell growth forms clones bearing

the same antigen-specific receptorsthe same antigen-specific receptors A naive, immunocompetent B cell is activated A naive, immunocompetent B cell is activated

when antigens bind to its surface receptors and when antigens bind to its surface receptors and cross-link adjacent receptorscross-link adjacent receptors

Antigen binding is followed by receptor-Antigen binding is followed by receptor-mediated endocytosis of the cross-linked mediated endocytosis of the cross-linked antigen-receptor complexesantigen-receptor complexes

These activating events, plus T cell These activating events, plus T cell interactions, trigger clonal selectioninteractions, trigger clonal selection

Fate of the ClonesFate of the Clones Most clone cells become antibody-secreting Most clone cells become antibody-secreting

plasma cellsplasma cells Plasma cells secrete specific antibody at the Plasma cells secrete specific antibody at the

rate of 2000 molecules per secondrate of 2000 molecules per second

Fate of the ClonesFate of the Clones Secreted antibodies:Secreted antibodies:

Bind to free antigensBind to free antigens Mark the antigens for destruction by specific or Mark the antigens for destruction by specific or

nonspecific mechanisms nonspecific mechanisms Clones that do not become plasma cells Clones that do not become plasma cells

become memory cells that can mount an become memory cells that can mount an immediate response to subsequent exposures immediate response to subsequent exposures of the same antigenof the same antigen

Immunological MemoryImmunological Memory Primary immune response – cellular Primary immune response – cellular

differentiation and proliferation, which occurs differentiation and proliferation, which occurs on the first exposure to a specific antigenon the first exposure to a specific antigen Lag period: 3 to 6 days after antigen challengeLag period: 3 to 6 days after antigen challenge Peak levels of plasma antibody are achieved in 10 Peak levels of plasma antibody are achieved in 10

daysdays Antibody levels then decline Antibody levels then decline

Immunological MemoryImmunological Memory Secondary immune response – re-exposure to Secondary immune response – re-exposure to

the same antigenthe same antigen Sensitized memory cells respond within hoursSensitized memory cells respond within hours Antibody levels peak in 2 to 3 days at much higher Antibody levels peak in 2 to 3 days at much higher

levels than in the primary response levels than in the primary response Antibodies bind with greater affinity, and their Antibodies bind with greater affinity, and their

levels in the blood can remain high for weeks to levels in the blood can remain high for weeks to months months

Active Humoral ImmunityActive Humoral Immunity B cells encounter antigens and produce B cells encounter antigens and produce

antibodies against themantibodies against them Naturally acquired – response to a bacterial or viral Naturally acquired – response to a bacterial or viral

infectioninfection Artificially acquired – response to a vaccine of dead Artificially acquired – response to a vaccine of dead

or attenuated pathogensor attenuated pathogens Vaccines – spare us the symptoms of disease, Vaccines – spare us the symptoms of disease,

and their weakened antigens provide antigenic and their weakened antigens provide antigenic determinants that are immunogenic and reactivedeterminants that are immunogenic and reactive

Passive Humoral ImmunityPassive Humoral Immunity

Differs from active immunity in the antibody Differs from active immunity in the antibody source and the degree of protectionsource and the degree of protection B cells are not challenged by antigensB cells are not challenged by antigens Immunological memory does not occurImmunological memory does not occur Protection ends when antigens naturally degrade in Protection ends when antigens naturally degrade in

the bodythe body Naturally acquired – from the mother to her Naturally acquired – from the mother to her

fetus via the placentafetus via the placenta Artificially acquired – from the injection of Artificially acquired – from the injection of

serum, such as gamma globulinserum, such as gamma globulin