Adaptive Immunity

Immunity: Third Line of Defense• Red bone Marrow Stem cells T and B cells• Specific reaction to microbial infection

Humoral Immunity Cell Mediated Immunity

B-cells

RecognizeSpecific Antigens

Make Antibodies against them

T-cells

RecognizeSpecific Antigens

Make Cytokines against them

Antigens & Antibodies• Antigens (Antibody generators)• proteins or large polysaccharides• Components of invading microbes• Non-microbial antigens:• pollen, egg white, serum proteins, blood cells etc

• Epitopes = specific region that interacts with Ab.

• Antibodies• globulin proteins (immunoglobulins) Y-shaped• Made in response to antigen; bind specifically to Ag• At least two identical sites that bind to epitopes• Bivalent molecule

Bivalent antibodies binding epitopes

Classes of ImmunoglobulinIgG:

• Y-shaped “Monomer”• readily cross vessel walls into inflammation site• ~80% of serum antibodies• Protects against bacteria and viruses• Neutralizes toxins• Enhances phagocytosis• Triggers complement• Confers immunity to fetus

AFM of a bivalent (monomer) antibody

Classes of Immunoglobulin

IgM:• Pentamer (can be monomer too)– Too big to cross into tissue from blood

• First Antibody response to 1 response• Dominates ABO blood group response• Effective in Complement Activation• Highly effective at Agglutination– Cross-links several Antigens

Classes of Immunoglobulin

IgA:• Dimer– Serum IgA (monomer)– Secretory IgA

• Mucus membranes and secretions– Mucus, tears, saliva, breast milk– Prevents pathogen attachment to mucosal surface– Colustrum• Decreases infant risk to GI infections

Dimeric IgA antibodies

Classes of Immunoglobulin

IgD:• Monomer• Blood, lymph, B-cell surface• No defined function

– 0.002% serum antibodies

IgE:• Monomer• Allergic Reactions, Parasitic Infections• Signals for complement and phagocytes• Binds mast cells/basophils histamine allergy

B-cell Activation

• Stem Cells B-cells– Each B-cell has surface Igs against specific Ag

• Binding of specific Ag activates THAT B-cell• Activated B-cell clonal expansion Plasma

cells Antibodies– Some activated B-cells become Memory cells

Antigen-antibody binding

• Antigen-Antibody Complex– Specific interaction• Affinity: strength of bond• Specificity: ability to distinguish minor differences in AA

– Binds at epitope– Complex formation tags foreign cells• Destruction by phagocytes and complement

Outcomes of Ab-Ag binding

• Agglutination– Clumping of Antigens

• Opsonization– Ab coats microbe– Enhances phagocytosis

• Neutralization– prevents Ag binding host cell

• Antibody-dependent cell-mediated cytotoxicity– Ab coats microbe– Ab binds T-cells/NK cells/other immune cells– Cytokines released lyse microbe

• Complement Activation– IgG, IgM– Binds C1– C1 C2, C4 C3 complementation cascade

T-cells and cellular immunity

• T-lymphocytes• Combat pathogens within host cells– Not exposed to circulating Antibodies

• Two Types– T-helper Cells (TH: TH1 and TH2)

– Cytotoxic T-cells (Tc)– Surface Receptors• Glycoproteins • CD (clusters of differentiation)

Antigen Presenting Cells

• B-cells• Phagocytes– Dendritic cells– Macrophages

• Chew up Microbe/ Antigen– Present parts of the Antigen on surface– Presentation involves a phagocytic receptor• MHC (major histocompatibility antigen)

The MHC-antigen complexMHC = major histocompatibility complex• Collection of genes that encode proteins found on all

nucleated mammalian cell membranes• Presence of MHC identifies the host

- Keeps immune system from making antibodies against host cells

Class II – found on APCs like B-cellsClass I – found on almost any cell of the host• Makes it possible for cytotoxic T-cells to attack

host cells that have been altered

T-Helper Cells (CD4+)- bind to MHC class II molecules

Activation of TH cells:1. TH cell recognizes an antigen in complex with MHC class II

presented on the surface of an APC2. TH cell proliferates and differentiates into TH1 and TH2

cells – secrete cytokines

TH1 = cytokines activate macrophages, enhance complementTH2 = cytokines stimulate production of antibodies important for allergic reactions, and eosinophils that protect against extracellular parasites

APC (dendritic cell) and TH cell

Antigen fragment

Antigen

MHC class II moleculesMicrobe

T helper cell

TH cell receptor contacting MHC-antigen complex

Cytotoxic T-cells (CD8+)• Recognize and kill altered or foreign cells• bind to MHC class I molecules

- found on all nucleated cells• Presented in complex with viral/parasitic antigens on

surface of infection-altered cells

Steps in destruction of target cells:1. Recognize foreign antigen/MHC class I protein

complex on cell2. Attaches and released perforin pore• Allows proteases to enter

3. Apoptosis = programmed cell death

Cytotoxic T-cells (CD8+)

1. Virus-infected cell with endogenous viral antigens (inside cell)

2. Abnormal antigen is presented on cell surface in complex with MHC class I molecules- TC cell with receptor for that antigen binds

3. TC cell induces destruction by apoptosis

AntigenMHC

class I

MHC-antigen complex

Cytotoxic T-cell

Apoptosis

Blebbing = external membranes bulge outward

Top: B-cell undergoing apoptosisBottom: Normal B-cell for reference

Antigen-presenting cells

Dendritic cells = Principle APCs to induce immune responses by T-cells• Long extensions = dendrites• Resemble nerve cell dendrites

1. engulf invading microbes2. degrade them3. transfer them to lymph nodes

for display to T-cells located there

Macrophages = (large eaters) • Innate immunity: important in

phagocytosis of apoptotic cells and other debris

• Adaptive immunity: become activated macrophages upon ingestion of foreign antigen- Appear larger and “ruffled”

1. Take in antigen2. migrate to lymph nodes3. present antigen to T-cells located there

Antigen-presenting cells

Extracellular killing

Natural killer cells = granular leukocytes that destroy virus-infected cells, tumor cells, and parasites• Part of innate immunity (non-specific)

- Not triggered by antigen• Remains external to target cell

Mechanism:1. Contact a target cell2. Determine if it expresses MHC class I self-antigen

(*tumor cells, viral-infected cells don’t)3. No expression induces lysis/apoptosis (similar to

that of cytotoxic T-cell)

Antibody-dependent-cell-mediated cytotoxicity

• Invaders too large to be phagocytized (euks) can be attacked by immune cells

• Uses antibodies of humoral system• NK cells, macrophages, neutrophils, and

eosinophils respond and kill targeted cells

Mechanism:• Target cell coated with antibodies• Immune cells bind to antibodies• Target cell is lysed by secretions

Antibody-dependent-cell-mediated cytotoxicity

Antibody-dependent-cell-mediated cytotoxicity

Eosinophils adhering to a parasite for external attack

CytokinesCytokines = chemical messengers of immune cells• Soluble proteins/glycoproteins• Produced by immune cells after a stimulus• Act only on a cell that has receptors for it

- Interleukins = cytokines that serve as communicators between leukocytes (WBCs)

- Chemokines = induce migration of leukocytes into areas of infection/tissue damage

- Interferons = protect cells from viral infection

Cytokines- Tumor necrosis factor (TNF) = cytokines that act in

inflammatory reactions; also target tumor cells- Hematopoietic cytokines = control development of

stem cells into red or white blood cells

Ex) Granulocyte-colony stimulating factor• Granulocyte precursors neutrophils

Cytokine storm = overproduction of cytokines- Damage to host tissues

Extracellular antigens

A B cell binds to the antigen for which it is specific. A T-dependent B cell requires cooperation with a T helper (TH) cell.

The B cell, often with stimulation by cytokines from a TH cell, differentiates into a plasma cell. Some B cells become memory cells.

Plasma cells proliferate and produce antibodies against the antigen.

Intracellular antigens are expressed on the surface of an APC, a cell infected by a virus, a bacterium, or a parasite.

A T cell binds to MHC–antigen complexes on the surface of the infected cell, activating the T cell (with its cytokine receptors).

Activation of macrophage (enhanced phagocytic activity).

The CD8+T cell becomes a cytotoxic T lymphocyte (CTL) able to induce apoptosis of the target cell.

B cell

Plasma cell

T cell

TH cell

Cytotoxic T lymphocyte

CytokinesCytokines

Lysed target cell

Cytokines activate macrophage.

Cytokines from the TH cell transform B cells into antibody-producing plasma cells.

Cytokines activate T helper (TH) cell.

Memory cell

Some T and B cells differentiate into memory cells that respond rapidly to any secondary encounter with an antigen.

Humoral (antibody-mediated) immune system Cellular (cell-mediated) immune systemControl of freely circulating pathogens Control of intracellular pathogens

Figure 17.20 The dual nature of the adaptive immune system.

Vaccines

• suspension of organisms/ parts of organism used to INDUCE immunity

• Artificial Active Immunity

Vaccine: Types

• Live attenuated whole-agent• Inactivated whole-agent• Toxoids• Subunit Vaccines– Recombinant subunit vaccines

• Nucleic acid Vaccines

Live attentuated Whole agent• living but attenuated (weakened) microbes

– Mutated virus– Related virus

• Attenuated viruses replicate in the body– Cell and humoral immunity

• Lifelong immunity• Counterindicated

– immune compromised– Attenuated microbes: from mutated strains can back-mutate to virulent

form

• Viral vaccines: MMR, Sabin polio, Smallpox, Flumist (influenza)• Bacterial vaccines: tuberculosis

Inactivated “Dead” Whole-agent

• Killed by formalin or phenol• Immunity not life-long– Boosters may be required– Primarily humoral response

• Examples:– Viral: Salk (polio, IPV), Rabies, Flu– Bacterial: Pneumococcal, Cholera

Subunit Vaccines• Highly immunogenic fragments

– Cannot replicate in host– Less side-effects/ dangers

• Recombinant vaccines– Desired Ag fragment expressed by unrelated, non-pathogenic microbe– Ex. HepB virion protein in GM yeast– Rabies glycoprotein in Vaccinia virus (V-RG)

• Toxoids– Tetanus, diphtheria– Several injections required for full immunity– Boosters every 10 years

• Conjugated Vaccines– Capsular polysaccharides: poor immunogens; T-independent Ags– Conjugate with Toxoid for maximal immunity– Ex. Hib

Nucleic Acid Vaccines (DNA vaccines)

• Newest “promising” vaccines• Plasmid DNA– Containing gene for immunogen of interest– Injected intramuscularly

• Gene gun• Conventional needle

– Expressed Protein Ag Red Bone Marrow humoral and cellular immunity• Long lasting immunity

• West Nile vaccine (horses)• Human trials underway

Types of Vaccine

Gene gun = DNA coated with gold or tungsten nanoparticles are “shot” into dermal cell cytosol• Inserted with glass micropipette

- Diameter smaller than cell- Punctures plasma membrane

• Eliminates• syringes/needles• refrigeration• lower costs

Recommended Immunization Schedule

Vaccine Development• Whole-agent vaccine– Grow in large amounts for use

• Early days– Smallpox scarified onto shaved calf bellies

• Cow “junk”

– Flu, Polio: grown in Eggs • Egg protein: allergen

– Human cells required• First HepB vaccine used Ags from chronically

infected as source

• Tissue cultureYolk sac

Allantoic cavity

Amniotic cavity

Chorioallantoic membrane

Current Vaccine Development

• Tissue Culture– Tissue slice– Digest with enzymes (trypsin)• Breaks down tissue into single cells

– Nutritive growth media• Cells adhere and divide to from a “monolayer”

– Infect with virus• CPE (cytopathic effect) caused by virus infection

Vaccine DevelopmentAdvancement in cultivation: cell cultureViruses may be grown in:Primary cell lines = derived from tissue slices; die out after a few generationsDiploid cell lines = develop from human embryos; maintained for ~100 generationsContinuous cell lines = (aka immortal cell lines)Cancerous cells; can be maintained indefinitelyEx) HeLa cell line• tend to have:

- Less round shape- Chromosomal abnormalities

Vaccine Safety: Risks v/s benefits• Disease caused by vaccine

– Smallpox• Variolation

– Incidence of disease decreased from 25% to 1%

– OPV (Sabin)• Poliovirus mutated• Reversion to wt

– Poliomyelitis

• Risk v/s Benefits– Public reaction

• Low perceived risk of contracting disease– Polio, measles

• Reports/ rumors of harmful effects– MMR autism– Flu Guillain Barre syndrome

• Herd immunity

Immune Disorders

Hypersensitivity

• Abnormal reaction to Antigen– Allergy– Sensitization to previous exposure to Allergen– Higher exposure to Antigen• Sensitized• Immune response to low levels of Ag

– Genetic predisposition

HypersensitivityHygiene hypothesis = sterile environments don’t provide enough stimulation for immune system

• Higher incidence in developed countries

• Eczema and hay fever less likely in children from larger families

• Allergies linked to antibiotic use in 1st year of life

• Asthma linked to use of household antibacterials

Hypersensitivity Reactions

• Type I: Anaphylaxis– Sytemic anaphylaxis (Anaphylactic shock)– IgE response

• Type II: Cytotoxic Reactions– IgM, IgG, complement response

• Type III: Immune complex reactions– IgG response against soluble Antigen

• Type IV: Delayed Hypersensitivity Reaction– Cell mediated response (CTL or ADCC)

Type I: Anaphylactic Reactions

• Rapid– 2-30 mins after exposure– Systemic

• Shock, breathlessness, can be fatal

– Localized• Hives

• IgE response– Binds basophils/ mast cells– Degranulation: release mediators

• Histamine• Leukotrienes• Prostaglandins

– Swelling, inflammation, runny nose, contraction of smooth muscles

Anaphylactic Reactions

Mediators: attract neutrophils and eosinophils to site of degranulated cell; and:Histamine• Increase vessel permeability

Swelling, redness• Smooth muscle contraction

Breathing difficulty

Leukotrienes & Prostaglandins• Not preformed in granules• Leukotrienes: prolonged smooth muscle contraction

asthmatic bronchial spasms• Prostaglandins: vasodilation, fever, pain

Systemic & Localized Anaphylaxis

• Systemic– Shock – Second or subsequent exposure to allergen

• Mediators vasodilation BP drop (shock)• Injected antigens (insect bites)

– Epinephrine• Constricts blood vessels

• Localized– Ingested or inhaled allergen

• Pollen– Inhalation

• Itchy eyes, runny nose, congestion, coughing, sneezing– Antihistamine (blocks histamine receptors)

– Ingestion• Food allergies• Hives, systemic anaphylaxis

Systemic & Localized Anaphylaxis

SEM of pollen grains, dust mite • common inhaled triggers of localized anaphylaxis

Ingestion: 8 foods = 97% food allergies• Eggs, peanuts, tree nuts, milk, soy, fish, wheat, peas

- 200 food allergy deaths per year in U.S.

Type II:Cytotoxic Reactions

• complement activation by IgG/ IgM with an antigenic cell

Ex) Transfusion reactions• RBCs destroyed by circulating antibodies

Type III: Immune Complex Reactions• IgG/ IgM against soluble antigens circulating in serum Immune complexes:• [Ag] > [Ab]• Complexes evade phagocytes• Soluble, circulating• “stuck” on capillaries, joints, organ tissues

• Activate complement: Transient Inflammation Attract neutrophils enzymes

- tissue destruction

Glumerulonephritis = inflammatory damage to kidney glomeruli

Type IV: Delayed Cell-Mediated Reactions

• T-cell activation• Development time: longer

– Days– T-cell and macrophage migration/ accumulation

• Sensitization– Macrophage phagocytoses Ag– Presents to T-cells– T-memory cells formed

• Subsequent exposure– Memory cells activated– Cytokines releases

• Attract and activate macrophages

Delayed Cell-Mediated Reactions

Ex) Allergic contact dermatitis = exposure to substances to which you have become extra sensitive• Fragrances• Metals• Plant oils (poison ivy)• Latex

Graft rejection Poison ivy plant

Catechols = oils secreted by poison ivy plant• Combine with skin proteins, become antigenic

immune response• First contact: sensitization• Second exposure: contact dermatitis

Comparison of the four types of hypersensitivity

Autoimmune Diseases

• Hosts immune response against self– Loss of discrimination between self v/s non-self– Thymic selection – >40 known ds., 75% women

• Autoimmune hepatitis– Hepatocytes display MHC-II to APCs• Viral infections (HepC, EBV)• Medications • Genetic predisposition

Autoimmune Diseases

• Immune complex reactions– Rheumatoid arthritis

• Immune complexes (IgG/ IgM) deposits in joint• Chronic inflammation• Damage to bone/ joint cartilage

– SLE (systemic lupus erythematosus)• Abs against cell components

– DNA– Tissue breakdown

• Cytotoxic autoimmune reactions– Graves Disease

• Abs that mimic TSH bind TSH-receptors• Increased production of thyroid hormones

– Hyperthyroidism– Goiter, bulging eyes

Immunodeficiencies

• Absence/ deficient immune response• Congenital

– DiGeorge’s syndrome (22q11.2)• Chromosome 22 • Defective/ missing thymus

– No CMI– Frequent/ severe infections

• Acquired: drugs, cancer, infectious agents– AIDS

• Final stage of HIV• Destruction of T-helper (CD4+) cells

– cancer, bacterial, viral, fungal, and protozoan diseases» Pneumocystis pneumonia, Kaposi’s sarcoma

• Diagnosis: CD4+ T-cell count below 200 cells/μl• Chemotherapy: inhibit viral enzymes

reverse transcriptase inhibitors

Types of Acquired Immunity: Active & Passive Immunity

Active immunity = acquired from an immune response to exposure of foreign antigens• Naturally acquired = exposure to antigens leads to

illness, recovery• Artificially acquired = vaccination

Passive immunity = acquired from transfer of antibodies from one person to another• Naturally acquired = mother to infant

- Transplacental, breast milk• Artificially acquired = injection of antibodies