Lecture 124, 125: Adaptive Immunity I and II

Immunoglobulins (Ig)

• soluble and humoral molecules with specificity to epitope
• one antigen can have multiple epitopes (bind to abs)

Antigen-Ab reactions

Immobilize (cross link: agglutination: IgM, IgA),
neutralize (IgG,IgA): like viruses
opsonize (phagocytosis): target for destruction
precipitin reaction: equal amounts of Ag-Ab until get a precipitate

Structure of Igs

4 polypeptides: 2 heavy chains and 2 light chains linked by S-S bonds

5 types of heavy chains: mu, gamma, alpha, delta, epsilon

Papain digestion results in

Fab (Ag-binding fragment):

• variable: binds epitope (both heavy + light)

Fc (constant region): heavy chain: (determines isotype)

IgM

Mu

• secreted as pentamer
• monomer on B cell surface
• 1st ab in adaptive immune response
  Functions:
• agglutination (immobilize), complement fixation, doesn't cross placenta

IgD vs IgE

IgD:

• Delta, monomeric
• BCR

IgE:

• Epsilon
• lowest in serum
• monomer
• adsorbed on mast cells and eosinophils (worms)
• immediate hypersensitivity (type I)

IgG

• Gamma
• most common in serum
• monomer (surface and excreted)
• 2nd ab released
• CROSSES PLACENTA
• Function: neutralize, opsonize, complement, hypersensitivity and Ab mediated cell cytotoxicity

IgA

• alpha
• monomer (serum)
• dimer (secretions)
• Most Ab produced daily
• Mucosal I (MALT)

Antibody dependent cell mediated cytotoxicity

Ab tagged bacteria, parasites, or worms attract NK cells and eosinophils

Type I hypersensitivity

immediate hypersensitivity

• IgE - mast cells and basophils
• Asthma, allergies, anaphylaxis
• not first time exposure, primed

Type II hypersensitivity

(antibody dependent cell mediated cytotoxicity)

• IgG/M interaction with cell membrane or ECM
• Complement and NK cells involved
• hemolytic anemia from mismatch
• bind to basement membrane - inflammation and tissue injury
• Abs block function (Grave's, MG, Rheumatic fever)

Type III hypersensitivity

immune complex

• circulating Ag-Ab complexes (IgG) - vasculitis
• Localized (Arthus): abscess or necrotizing vasculitis

Systemic: serum sickness

• Exogenous: non humans Abs/drug induced
• Endogenous: SLE

MHC molecules

HLA

• I, II
  III (complement)

MHC I vs II

MHC I:

• in all nucleated cells
• self reporting: is it viral infected or tumor
• HLA A,B,C
• compatibility important for transplants

MHC II:

• expressed mostly on APCs (DC, macrophages)
• B cells, active T cells, cells in thymus and intestine
• involved in antigen presentation

TCR

• heterodimer (alphaB or GammaDelta)
• associated with CD3 (signal transduction)
• MHC restricted: bind to peptide fragments associated with MHC I and II (stabilized by CD4,8)

Cellular interactions

Cytokines: low MW soluble messengers
Chemokines: chemoattractant cytokines
Adhesion molecules: integrins and selectins

CD:

• indicator of function
  CD 3,4,8

Lymphocyte activation

Main cell of adaptive IR: T helper cell
Antigen presentation (I or II) required for lymphocyte activation since TCR is MHC restricted

Processing of MHC II

• main APC: dendritic cells- sit in lymph nodes and make sure everything is ok (sentinel)
• Actin dependent phagocytosis
• PRRs engage PAMPs (EC antigens)
• Ab or C tagged ag
• Ag displayed on MHC II (can display self or non self)

MHC I presentation (processing)

intracellular Ags (bacteria, virus) - tagged with ubiquitin - proteasome destruction - ER - Golgi - display on MHC I - recognition by CD8+ (T cytotoxic)

T cytotoxic recognize tumor + virus infected cells

Immunological synapse of T helper Cell

ONLY RECOGNIZE MHC II associated antigens

interface between naive Th and APC

• stabilized by CD28 (costimulatory) - signal transduction
• absence of signal: anergy/apoptosis

T helper cell activation

1st contact with Ag - prime T helper (naive)- cytokines - det which T cell population to differentiate into

Some become memory cells - high expression of CD28-

when one T cell activates, cytokine blocks other T cells from differentiating

T Helper cell populations

T-helper → Th1, Th2 or Th17 (cytokines from APC)

1. Th1: IFN-gamma - IL-12
2. Th2: IL-4/IL-10
3. Th17: IL-6/TGF-B

CD8+ cell activation

Recognize Ags on MHC I - expression of IL2R

• With help from CD4+ cells - T-cytotoxic - engage antigens from multiple cells (intracellularly)

Perforin (makes holes) and granzyme (apoptosis)

B cell activation

recognize antigens with and without MHC II
Ag binds to BCR - endocytosis - Ag presentation on MHC to CD4+ (T dependent Ags) - Th2/17 (IL4/17) - plasma cells (some memory)

T independent antigens

activate B cells without help from T cells

• non protein antigens (lipids, polysaccharides)
• effective against capsulated bacteria

Ti1: bind to LPS, B cell mitogens
Ti2: multivalent polysaccharides: stimulate only mature B cells

2 arms of Adaptive IR

regulated and activated by helper T cells

Humoral IR:

• abs (soluble in plasma)
• extracellular pathogens,
  hypersensitivities: Type I,II,III

CMI:

• intracellular
• Type IV hypersensitivities (T cytotoxic cells)

Type IV hypersensitivity

Delayed Type hypersensitivity

• Th1/macrophage induced inflammation due to cytokines
• takes time
• Ag injected or DTH (delayed type hypersensitivity): 48 to 72 hours: due to microbes
• (mycobacteria, HBV,HCV, fungi)
• can do testing to see whether been exposed to microbe

Immunologic memory

Primary: exposure

• start with IgM and then go to IgG after 2 weeks
• isotype switching

2ndary: memory

• only IgG
• vaccines

Affinity maturation (hypermutations):

• in secondary immune response
• antibodies go through mutations over time for better binding to antigen

Immune regulation in relation to tolerance

Anergy (CD28/B7)
b) Treg (CD4, CD25 & FOXP3): suppresses immune system
c) CD8+ suppressor cells (graft rejection autoimmune diseases)

Immune regulation in relation to proinflammatory markers and Th1/Th2 paradigm

Proinflammatory: Th17 (antagonists of IL-17 very effective in psoriasis)
• Th1/Th2 paradigm: when Th1 active, Th2 inhibited

The epitope-binding site on the antibody is located

on the variable region

• The epitope-binding site of an antibody is located at the tip of the variable region of both the heavy and light chains.

• This area is highly specific and undergoes somatic hypermutation to improve binding through affinity maturation.

• The variable region is part of the Fab fragment, which includes both variable and part of the constant regions—but only the variable region actually binds the antigen.

Which of the following is initiated by the interaction of host cell membranes and IgM/IgG but never IgE?

Type II hypersensitivity

Type II hypersensitivity is:

• Antibody-mediated cytotoxic hypersensitivity

• Caused by IgM or IgG binding to antigens on host cell membranes (e.g., RBCs, basement membrane)

• Leads to complement activation, inflammation, opsonization, or ADCC (antibody-dependent cell-mediated cytotoxicity)

Examples:

• Hemolytic anemia (transfusion mismatch)

• Goodpasture syndrome

• Graves disease, Myasthenia gravis (non-cytotoxic Type II variants)

Why Other Choices Are Incorrect:

• Type I hypersensitivity: Mediated by IgE → mast cell degranulation (e.g., anaphylaxis, asthma)

• Type III hypersensitivity: Involves immune complexes (Ag-IgG) depositing in tissues → vasculitis, serum sickness

• Arthus reaction: A localized Type III hypersensitivity, not Type II

Mnemonic for heavy chains (isotypes): "MAGDE"

Mu (μ, IgM), Alpha (α, IgA), Gamma (γ, IgG), Delta (δ, IgD), Epsilon (ε, IgE)

Ti-1 Antigens

• Function as polyclonal B-cell activators (mitogens).

• Bind to non-BCR receptors, e.g., LPS (lipopolysaccharide).

• Can activate immature and mature B-cells broadly.

Ti-2 Antigens

• Usually repetitive polysaccharides (e.g., capsules).

• Multivalent → cross-links BCRs.

• Stimulate only mature B-cells.

• Produce mainly IgM, little to no memory.
  

Inflammation Steps

• "Vasodilation (histamine) - Increased permeability - Leukocyte migration (via chemokines)

• Phagocytosis and tissue repair"

Classical Pathway Key Components

C1, C2, C4

Alternative Pathway Key Components

C3, Factor B/D

Lectin Pathway Key Components

MASPs, C4, C2

Complement Deficiency (e.g., C3)

"Impaired opsonization and bacterial clearance

Recurrent encapsulated bacterial infections

Chronic Granulomatous Disease (CGD)

- "NADPH oxidase deficiency → no ROS - Recurrent catalase+ bacterial/fungal infections

NK Cell Deficiency

- "Failure to kill virus-infected cells -Severe viral infections

PRR/TLR Deficiencies

"Impaired recognition of pathogens

Broad susceptibility to infections

Mnemonic for Complement Deficiency Infections: "Some Nasty Killers Have Pretty Nice Capsules"

Strep pneumo, Neisseria, Klebsiella, H. influenzae, Pseudomonas, N. meningitidis, Cryptococcus

Tolerance

Prevention of immune response to self-antigens

Anergy

T-cell unresponsiveness due to lack of co-stimulation (CD28/B7 absent)

T-regulatory (Treg) cells

CD4+, CD25+, FOXP3+ cells suppress immune responses; prevent autoimmunity

CD8+ Suppressor T cells

Help in graft acceptance and control autoimmunity

Th1/Th2/Th17 Cross-Inhibition

Cytokines from one Th type inhibit differentiation of others (e.g., IFN-γ inhibits Th2/Th17)

Which of the following best describes where D (diversity) gene segments are located in the immunoglobulin genome?

A. Constant region of both light and heavy chains

B. Constant region of heavy chains but not light chains

C. Variable region of both light and heavy chains

D. Variable region of heavy chains but not light chains

E. Variable region of light chains but not heavy chains

D

The correct answer is variable region of heavy chains but not light chains (D). The variable regions are responsible for the specificity of the antibody (and of the B-cell receptor of which the antibody is part) to undergo V(D)J recombination. This does not occur in the constant regions (A, B). Light chains contain only V and J segments (C, E).

A patient is found to have B cells that are unable to undergo affinity maturation. Which of the following consequences is most likely?

A. Although the B cells would be able to identify antigens, the secondary immune response will be weak.

B. B cells would be decreased in the patient’s blood.

C. Only immature B cells would be present in the patient’s blood.

D. The B cells would be unable to identify antigens.

E. V(D)J recombination would be reduced in the B cells.

A

The correct answer is although the patient’s B cells can identify antigens, the secondary immune response will be weak (A). Without affinity maturation, B cells will still be able to bind antigens, but they will not have a survival advantage. B cells rely on T-cell and antigen-presenting cell interactions to proliferate and survive. This will weaken the humoral immune response on re-exposure to the antigen. The B cells would still leave the bone marrow as mature naive B cells, so the number would not decrease (B). Immature B cells should not be found in the blood and do not undergo affinity maturation (C). The B cells would still be able to identify antigens (D). V(D)J recombination is a separate process that achieves antigen diversity in B cells and would not be affected (E).

Which of these statements best compares V(D)J recombination and somatic hypermutation?

A. Both occur in the bone marrow.

B. V(D)J recombination is key to affinity maturation, while somatic hypermutation is required for increased antibody diversity.

C. V(D)J recombination occurs in mature naive B cells, while somatic hypermutation occurs in immature B cells.

D.V(D)J recombination occurs prior to antigen exposure, while somatic hypermutation occurs after antigen exposure.

D

The correct answer is V(D)J recombination occurs prior to antigen exposure, while somatic hypermutation occurs only after antigen exposure (D). V(D)J recombination occurs during the formation of antibodies in pre-B cells and immature B cells, not mature naive B cells (C), thus occurring well before antigen exposure; somatic hypermutation occurs after an antigen is identified by a mature naive B cell, prompting it to move to the germinal centers of lymphoid follicles, not the bone marrow (A), and undergo somatic hypermutation. V(D)J recombination is key to affinity maturation while somatic hypermutation is required for increased antibody diversity (B) is flipped: V(D)J recombination results in a more diverse collection of antibodies, and somatic hypermutation allows for antibodies with higher-affinity binding.

Which of the following enzymes is most responsible for the recognition and cleavage of DNA during V(D)J recombination?

A. Caspase 1

B. DNA polymerase I

C. DNA polymerase II

D. Recombination-activating gene 1 (RAG-1)

E. Terminal deoxynucleotidyl transferase (TdT)

D

The correct answer is recombination-activating gene 1 (RAG-1) (D). RAG-1 is a key player in V(D)J recombination. RAG-1 and RAG-2 recognize and cleave DNA in areas that flank variable (V), diversity (D), and joining (J) gene segments, allowing for the recombination of these gene segments and leading to the formation of a unique and diverse repertoire of B- and T-cell receptors. The other options are not involved in V(D)J recombination. Caspase 1 (A) cleaves proteins like interleukins. DNA polymerase I (B) and II (C) are important in normal DNA replication. TdT is important in the creation of junctional diversity by adding nucleotides randomly to the gene, but it is not involved in V(D)J cleavage steps (E).

By which of the following mechanisms does a CD4⁺ helper T cell become activated by its antigen?

A. TCR and CD4 co-receptor bind antigen presented on MHC class I

B. TCR and CD4 co-receptor bind antigen presented on MHC class II

C. TCR and CD4 co-receptor bind soluble antigen

D. TCR binds antigen presented on MHC class I

E. TCR binds antigen presented on MHC class II

B

The correct answer is TCR and CD4 co-receptor bind antigen presented on MHC class II (B). T-cell activation requires both presentation of antigen by an MHC molecule on the cell surface and a costimulatory signal from the co-receptor; in this case, activation of a CD4⁺ T cell would require both antigen presentation via MHC class II and a costimulatory signal from the CD4 co-receptor. TCR and CD4 co-receptor bind antigen presented on MHC class I (A) is incorrect because CD4⁺ T cells do not bind or recognize antigen presented on MHC class I; CD8⁺ T cells bind MHC class I. TCR and CD4 co-receptor bind soluble antigen (C) is incorrect because TCRs do not bind soluble antigen. In contrast with the antigen receptor of a B cell (B-cell receptor or immunoglobulin), which can recognize and bind soluble antigen, the TCR requires antigen to be presented by an MHC molecule on the cell surface. TCR binds antigen presented on MHC class I (D) is incorrect because CD4⁺ T cells bind to MHC class II. TCR binds antigen presented on MHC class II (E) is one step of activation, but a CD4⁺ T cell will not be strongly activated without the costimulatory signal from the CD4 co-receptor.

Which cell surface protein is expressed on the surface of virtually every nucleated cell in the human body and is crucial for virus eradication by the immune system?

A. CD3

B. CD8

C. MHC class I

D. MHC class II

E. T-cell receptor

C

The correct answer is MHC class I (C), which is expressed on all nucleated cells and presents intracellular proteins to CD8⁺ T cells. CD8⁺ effector T cells kill virus-infected cells, which present viral antigens via the MHC class I molecule. CD3 (A) is a cell surface receptor found on all T cells and is part of the TCR complex, responsible for intracellular signaling. CD8 (B) is a T-cell co-receptor that facilitates interaction between naive T cells and the MHC class I molecule of target cells during T-cell activation. MHC class II (D) is expressed only on the professional antigen-presenting cells and presents extracellular antigens to CD4⁺ T cells. The T-cell receptor (E) is present only on T cells and only recognizes processed antigens bound to MHC molecules (peptide–MHC complexes) on cell surfaces.

Which of the following is responsible for the genetic diversity of MHC molecules?

A. Expression of only one copy of parental MHC genes

B. Genetic recombination

C. Multiple distinct MHC gene loci

D. Unique MHC alleles in the population

D

The correct answer is unique MHC alleles in the population (D). The abundance of unique HLA alleles (estimated to be >14,000) in the MHC complex of genes (a single locus) is responsible for the diversity of MHC molecules. The great diversity of different MHC alleles is known as genetic polymorphism, and the MHC complex of genes is the most genetically diverse locus present in the genome. Expression of only one copy of parental MHC genes (A) is incorrect, because the MHC genes are expressed co-dominantly, meaning that MHC genes inherited from parents are all expressed equally. Genetic recombination (B) is responsible for the genetic diversity in antigen receptors, such as the T-cell receptor. The MHC genes do not undergo genetic recombination. Multiple distinct MHC gene loci (C) is incorrect because the MHC complex of genes is nearly always located on a single locus on chromosome 6.

A 12-year-old male is rushed to the emergency department when he develops difficulty breathing, hives, and eyelid swelling after eating seafood. This reaction most likely involves which of the following immune responses?

A. C1 inhibitor deficiency

B. CD4⁺ T cell–mediated response

C. IgE cross-linking and mast cell degranulation

D. Immune complex deposition within the bronchioles

E. Spontaneous basophil activation

C

The correct answer is IgE cross-linking and mast cell degranulation (C). This patient is experiencing an allergic reaction due to a type 1 hypersensitivity food allergy. This is demonstrated by his dyspnea, resulting from histamine and leukotriene-induced bronchoconstriction, as well as hives and eyelid edema, attributed to histamine-induced increased vascular permeability at the level of the skin. C1-inhibitor deficiency (A) causes hereditary angioedema, but not hives or wheezing. CD4⁺ T cell–mediated response (B) is associated with type 4 hypersensitivity, which takes 24-72 hours to present and would not induce wheezing or hives. Immune complex deposition within the bronchioles (D) is a type 3 hypersensitivity seen in immune complex rheumatologic diseases. The described reaction is indicative of IgE cross-linking causing mast cell degranulation rather than a random, spontaneous activation of basophils (E).

A 40-year-old female visits her health care provider reporting headaches and double vision for the past 2 weeks. Additionally, she has been feeling extremely tired and has observed that her symptoms tend to intensify as the day progresses. During physical examination, both of her eyelids are drooping. She is diagnosed with myasthenia gravis. Which of the following mechanisms is most responsible for her immune disease?

A. Complement-mediated destruction of myocytes

B. Immune-mediated phagocytosis of neurons

C. Increased immune complex formation

D. Inhibitory binding of antibodies to receptors

E. Release of IgE

D

The correct answer is inhibitory binding of antibodies to receptors (D). Myasthenia gravis (MG) is an example of one type of type 2 hypersensitivity in which antibodies bind to and inhibit neuron receptor function. Specifically, acetylcholine receptors at the neuromuscular junction are targeted, inhibiting the transmission of signals from nerves to muscles and leading to lack of effective neuron impulse transmission. MG is not mediated by complement destruction of myocytes (A) or due to phagocytosis of neurons (B), which are mechanisms more typical of the complement-induced cell death seen in type 2 hypersensitivity reactions causing transfusion reactions. Additionally, it does not involve immune complex formation (C), as in type 3 hypersensitivity, or IgE release (E), as in type 1 hypersensitivity.