Biosystems II exam 5:

Steps of B cell activation/Antibody secretion:

1. Mature, Naive BCell in peripheral lymphoid organ recognizes Ag by BCR (IgM/D)

2. (TCell and other stimuli) - Clonal expansion of BCell

3. BCell proliferation and diffferentation

4. Turn into plasma cell and secrete IgM, IgG, memory BCells (Secretion, isotope switching, affinity maturation)

What is TCell dependent BCell activation? (TD)

Ag has a protein component (can cross-link Rs), activates in lymph node follicles and needs help Isotope switching, Affinity maturation, Plasma cells and inducing memory B cells

What is TCell independent BCell activation? (TI)

Ag with multiple identical/repeating epitopes (can cross-link Rs), activates in marginal zone of spleen and DOES NOT need help with functions (only need signal 1)

Example of a Thymus dependent agent (TD)

Protein Ag binds MHC class II molecule - Isotope switching, affinity maturation and memory BCell response occurs

Example of a Thymus independent agent (ID)

Polymeric antigens (polysaccharides, glycolipids and nucleic acids) CANNOT bind MHC but may bind multiple BCRs to activate but with little to no isotope switching, affinity maturation and memory cells

Plasma cells are…

Terminally differentiated, activated BCell; high rate of Ig production

2 types of plasma cells:

• Short lived: TI Ag - white pulp of spleen/nonlymphoid tissue

• Long-lived: TD Ag - migrate to bone marrow and constantly secreting Ig months to years following Ag exposure

What does cross-linking do to BCRs?

Initiates activation cascade when 2+ BCRs cross linked OR BCR + complementR/TLR

Signal 1 and 2 in TD activation:

• Signal 1: Protein antigen bind BCR and associated with Iga/IgB heterodimer - Ag presented on MHC class II

• Signal 2: Interaction between CD40 on BCell and CD40L on helper TCell

Signal 1 and 2 in TI activation:

• Signal 1: Non-protein multivalent Ag binds BCR associated with Iga/IgB heterodimer

• Signal 2: Complement R or TLRs = innate immunity = facilitates adaptive immunity (aka BCell activation)

BCells are professional…

APCs

Steps of BCell Ag presentation:

1. Ag binds BCR by PRR/TLR and MHC class I/II (usually class II)

2. Ag internalized, processed and presented to ACTIVATED T helper cells (Naive TCell previously activated by same Ag on DC)

In TD BCell activation, the BCell and TCell recognize…

the same Ags so the adaptive response is microbe-specific (recognize different antigenic epitopes from same microbe)

Helper TCell dependent response (TCell POV):

1. Ag presented by DC to naive TCell (peptide ; B7+CD28)

2. TCell now activated and secretes IL-2, CD40L and Th1/2 differentiation

3. Activated TCell migrates toward BCell follicle

Helper TCell dependent response (BCell POV):

1. BCR recognized protein Ag (captured and processed)

2. Migration of Ag-stimulated BCell towards T cell zone (for presentation)

3. Activation of BCell with help from activated TCell (Peptide ; CD40 (B) + CD40L)

Where does TD BCell activation occur?

The interface of the BCell follicle and TCell zone in secondary lymphoid organs

What are “germinal center reactions“?

Affinity maturation and Isotope class switching (done in a special part of lymph node/spleen)

What enzyme is involved in Affinity maturation/ Somatic Hypermutation?

Activation-induced deasminase (AID)

TD BCells REQUIRE what to induce AID expression (for affinity maturation and isotype switchingg)?

costimulation (signal 2; CD40 + CD40L)

What is the function of AID in affinity maturation/Somatic hypermutation?

Changes C to U and generates point mutations in variable regions (change affinity that increases with each exposure)

As exposure to Ag increases, Affinity of BCR to Ag..

Increases (increased ability to bind at lower concentration)

Isotope switching is…

Producing the same specificity for Ag as membrane-bound BCRs w/ switched BCR

Isotope switching occurs in response to which different cell/cytokines?

• TH1 = IFN-y

• TH2 = IL-4, IL-5 and IL-13

Where does isotype switching occur?

Peripheral lymphoid organs (activated BCells only, in follicles/germinal center)

DC activates TCell then TH1 TCell activates BCell with what cytokine/switch/function?

IFN-y causes switch to IgG which has FcR-d phagocytic response, complement activation and neonatal immunity effects

If TCell does NOT activate BCell, what signaling/functions occur?

IgM+ BCell sends no signal for switching and activates complement (innate)

DC activates TCell then TH2 TCell activates BCell with IL-4, what switch/function?

Causes switch to IgE which helps in helminth immunity and mast cell degranulation (hypersensitivity)

DC activates TCell then TH2 TCell activates BCell with IL-5, what switch/function?

Causes switch to IgA which plays a role in mucosal immunity

Bacteria with a polysaccharide rich capsule will cause what reaction?

TI - induce IgM = complement activation

A virus/bacteria enters what reaction is caused?

TD - IFN-y induced by TH1 = IgG

A helminth/parasite enters, what reaction occurs?

TD - IL-4 induced by TH2 = IgE

An infection in mucousal area occurs, what reaction follows?

TD - IL-5 induced by TH2 = IgA

Primary Humoral response, Lag, Peak, Isotype and affinity:

• 7-10 days

• Low peak

• IgM (no TCell)

• Low

Secondary Humoral response, Lag, Peak, Isotype and affinity:

• 3-5 days

• >100x higher

• IgG (IgE/IgA)

• High

The humoral immune response is mediated by…

secreted antibodies

The humoral immune response (Ig secretion) gives immunity against..

extracellular microbes and toxins

Humoral immunity includes components of both…

Active and Passive immunity

Humoral protectiveness can come from…

vaccines

Harmful humoral immunity is caused by..

Auto-antibodies

What is the primary Ig in the blood?

IgG

What is the primary Ig in mucosal organs?

IgA

What is antibody neutralization?

Ig bonds virus/toxin before it can bind to cells = prevents infection/attachment

Examples of antibody neutralization:

• HIV-go120 binds CD4

• Influenza-HA binds sialic acids

• Tetanus binds neurotoxin

• Cholera activates AC

2 things that function as opsonins? (promote phagocytosis via Fc-R)

• Antibodies

• Complement proteins

Fc-R specificity…

• Fcy-R = IgG

• Fca-R = IgA

• FcE-R = IgE

How does opsinization/Fc-R work?

Fc-R associates with signaling molecules (ITAMs and ITIMs)

ITAMs recruit…

kinases

ITIMs recruit…

Phosphotases

Steps of Fc-R mediated phagocytosis:

1. Opsonization of microbe by IgG

2. Opsonized microbe binds by IgG and Fcy-R

3. Fc-R signals to activate phagocytosis

4. Phagocytosis and killing of microbe by ROS and NO (IFN-y from TH cell = kill signal)

What is Antibody-Dependent Cellular Cytotoxicity (ADCC)

Antibody-coated cell destruction by release of granules by IgG binding and NK cell recognition = granzyme and perforin

What is the role of complement in host defense:

Circulating cell membrane proteins are activated during innate immunity in absence of Ig (alternative pathway) AND/OR in presence of Ig during adaptive immunity (classical pathway) to opsonize, form MAC and inflammation/recruitment

Complement proteins are involved in antibody-mediated tissue injury AKA

Hypersensitivity reactions

Functions of complement: (3)

• Opsonize microbes to promote phagocytosis

• Form polymeric protein complex (MAC)

• Promote inflammation at the site of complement activation

Functions of IgA:

• Mucosal immunity (GI and upper respiratory)

• Transport across epithelium (to get to lumen/breastmilk)

• Acts as poly-Ig R

• Neutralization in the lumen

Functions of IgG:

• Neonatal immunity (transport across placenta by FcRn)

• Neutralization (most secreted)

• Opsonization

• ADCC

• Complement (classical)

IgE functions:

Eosinophil, mast cell activation and helminth defense

IgM functions:

• Complement activation (classical)

• BCR

IgD functions:

BCR only

ABO phenotypes:

• Type A (Ag-A and Ig-B)

• Type B (Ag-B and Ig-A)

• Type AB (Ag-A and B so NO Ig) (universal recipient)

• Type O (Ig- A and B so NO Ag) (universal donar)

Type A has self Ag and Ig-B so who CAN and CANNOT donate?

• CAN receive: A and O

• CANNOT receive: AB or B

Type B has self Ag and Ig-A so who CAN and CANNOT donate?

• CAN receive: B and O

• CANNOT receive: A or AB

Type AB has self Ag and NO Ig so who CAN and CANNOT donate?

• CAN receive: A, B, AB and O (universal recipient)

Type O has NO Ag BUT Ig-A and B so who CAN and CANNOT donate?

• CAN receive: O only

• CANNOT receive: A, B, AB (universal donor)

Rh in pregnancy:

• Rh+ father

• Rh- mother carrying first Rh+ fetus

• Rh+ fetus makes self Ag - cross to mother

• Mother produces anti-Rh+ Ig (non-self)

• Mothers 2nd pregnancy with Rh+ fetus = fetal damage by anti-Rh+ Ig

SLE/Lupus

1. Immune complexes are deposited in wall of blood vessel

2. Presence of immune complexes activates complement and attracts inflammatory cells such as neutrophils

3. Enzymes released from neutrophils cause damage to endothelial cells of basement membrane

What is a hypersensitivity reaction defined as…

An excessive, exaggerated and/or abnormal immune response that causes tissue injury

Symptoms of a hypersensitivity reaction can manifest as…

• Immediate

• Delayed

• Chronic

• Autoimmune

2 ways that hypersensitivity occurs:

• Response to foreign Ag may be dysregulated or uncontrolled leading to tissue injury

• Immune responses may be directed against self Ag (due to failure of self-tolerance)

What is autoimmunity?

Responses against self Ag leading to autoimmune diseases

4 types of hypersensitivity reactions:

• Type I: immediate by TH2

• Type II: Ig-mediated from auto-reactive Ig

• Type III: Immune complex mediated by Ag-Ig complexes

• Type IV: TCell mediated by TH1 Cells

Type I hypersensitivity is….

• Immediate

• TH2 mediated (IgE)

• Mast cells cause histamine, LT and Cytokine release

• Eosinophil and Neutrophil inflammation

What is Atopy:

Tendency to produce an exaggerated IgE response to harmless substances (can have increased genetic risk aka “atopic individual“)

Mast cells express elevated levels of FcE-R (IgE R) which causes…

hyperreactive airway manifestation

Steps of allergen exposure (general)

1. Allergen exposure (Pollen)

2. Allergens registers as a foreign “threat“

3. Immune system produces IgE and mast cells

4. Allergy symptoms: runny nose, itchy eyes and sneezing

Examples of allergens/triggers:

Food, pollen, venom, dander, drugs, pollutants, exercise, temperature

Immediate hypersensitivity chain of events: (11)

1. Primary allergen exposure

2. Allergen presentation to allergen-specific naive CD4 TCell

3. Differentiation TH2 (IL-2 and IL-4)

4. Activation of allergen-specific BCell (IL-5 and IL-13)

5. IgE class switch

6. Sensitization of mast cells

7. Secondary allergen exposure (binds IgE made by previous exposure)

8. Mast cell activation/Fc-crosslinking (by FcE-R)

9. Release mediators (degranulation of mast cells = vasoactive amines/Histamines and lipid mediators/PG+LT)

10. Late phase cytokine release (2-8 hours after repeat exposure to allergens)

11. Response occurs in minutes AKA immediate

______ exposure is required for activation of ‘sensitized‘ mast cells

repeat

Mast Cell Mediators:

• Histamines: vascular permeability, smooth muscle contraction, vascular dilation (edema)

• LT: vascular permeability, smooth muscle contraction and mucus secretion

• PG: vascular dilation (blood flow)

• Protases (tryptase): local tissue damage

Histamine and lipid mediators are _____ while cytokines are NOT

premade

Cytokines involved in mast cell functions (late phase):

• IL-4: TH2

• IL-5: eosinophil activation

• IL-13: mucus secretion

• TNFa: increased leukocyte recruitment

Histamines cause

Vascular permeability, smooth muscle contraction, vascular dilation (edema)

Leukotrienes cause:

Vascular permeability, smooth muscle contraction and mucus secretion (Bronchoconstriction and GI hypermobility)

Inflammation mediators of hypersensitivity:

• TNFa

• IL-4/5/13

• LT

• PG

Which mediators cause tissue damage?

Tryptase and mast-cell proteases

Inhaled (type I) hypersensitivity syndromes:

• Allergic rhinitis and Sinusitis (hay fever): Increased mucus and inflammation of upper airways/sinuses

• Bronchial asthma: bronchial hyper-responsiveness caused by SM contraction, inflammation and tissue injury from late phase cytokines

Ingested (type I) hypersensitivity syndrome:

Food allergies (increased peristalsis due to contraction of intestinal muscles)

Systemic (type I) hypersensitivity syndrome:

Anaphylaxis (drugs/bee/food, Fall in BP (shock) caused by vascular dilation and airway obstruction due to laryngeal edema

Treatment for Type I hypersensitivity: Various allergic diseases

• “Desensitization” (repeated admin of low doses of allergens) - Unknown, may inhibit IgE production and induce TCell tolerance

• Anti-IgE antibody - Neutralize and eliminate IgE

• Antihistamines - Blocks actions of histamine on vessels and SM

• Cromolyn - inhibits mast cell degranulation

Treatment for Type I hypersensitivity: Anaphylaxis

Epinephrine (SM contraction, increase CO, inhibit mast cell degranulation)

Treatment for Type I hypersensitivity: Bronchial asthma

Corticosteroids and PI’s (Reduce inflammation and relax bronchial SM)

Singulair is a ______ that inhibits _____

LTa ; Bronchoconstriction

-mabs for type I hypersensitivity MOA:

Binds Fc portion of IgE to prevent binding to FcE-R

Corticosteroids and cromolyn MOA:

Prevent cytokine production and mast cell degranulation = no inflammatory cytokines or PG/LT

What are type II hypersensitivity reactions?

Antibodies are reactive to self tissue where normal Ig-mediated effector functions lead to cellular injury or dysfunction (complement activation, Fc-R activation, inflammation)

Type II Antibody-mediated diseases:

• Autoimmune hemolytic anemia

• Goodpasture syndrome

• Myasthenia gravis

• Graves disease

Autoimmune hemolytic anemia (Type II):

• Target: Erythrocyte membrane proteins (Rh blood group Ag)

• Mechanism: Opsonization and phagocytosis of erythrocytes

• Manifests: Hemolysis and anemia

Goodpasture syndrome (Type II):

• Target: Noncollagenous protein in basement membranes of kidney glomeruli and lung alveoli

• Mechanism: Complement and Fc-R mediated inflammation

• Manifests: Nephritis and Lung hemorrhages

Myasthenia gravis (type II):

• Target: ACh R

• Mechanism: Ig inhibits ACh binding; down-modulates R’s

• Manifests: Muscle weakness/paralysis

Graves disease (type II):

• Target: TSH R

• Mechanism: Ig-mediated stimulation of TSH R’s

• Manifests: Hyperthyroidism

What is Type III hypersensitivity?

Occurs when circulating immune complexes deposit in blood vessels and cause complement- and Fc receptor–mediated recruitment and activation of leukocytes, especially neutrophils, leading to inflammation and vasculitis