7: Humoral Immune Responses

What are humoral immune responses?

Adaptive immune responses mediated by antibodies produced by activated B lymphocytes

What does the BCR recognize?

A specific epitope on an antigen.

What happens during BCR engagement?

Antigen binds membrane-bound immunoglobulin on the B-cell surface.

What is BCR cross-linking?

• B Cell Clustering

The simultaneous binding of multiple BCR molecules by antigen, bringing receptors together and initiating signaling.

What two types of receptors bind the same antigen?

B-cell receptors and complement receptors.

What does the complement receptor recognize?

Complement fragments attached to the antigen.

What connects complement activity to B-cell activation?

Complement fragments deposited on antigens interact with B-cell complement receptors.

What is CR2 on a B cell?

CR2 (CD21) is a complement receptor that acts as a B-cell co-receptor

What does CR2 bind?

C3d, a fragment of complement protein C3 attached to an antigen.

Why is C3d deposited on an antigen?

It delivers a strong co-stimulatory molecule that leads to B cell activation!

What three proteins form the B-cell coreceptor complex?

• CR2

• CD19

• CD81

What is the function of CD19?

It contributes to amplification of intracellular B-cell activation signaling

What is the role of CD81?

It is a component of the B-cell coreceptor complex that supports coreceptor function

Which BCR-associated proteins transmit antigen receptor signals?

Igα and Igβ.

How can TLR signaling contribute to B-cell activation?

TLR recognition of microbial PAMPs provides additional activation signals alongside BCR engagement

Which two immune cells interact during T-cell-dependent B-cell activation?

A helper T cell (Th cell) and a B cell

Which receptor is expressed on the B cell during T-cell-dependent B-cell activation?

CD40

Which ligand is expressed on the helper T cell?

CD40 ligand (CD40L)

What interaction provides an essential helper T-cell signal to B cells?

CD40 on the B cell binding CD40L on the helper T cell.

What must a B cell recognize before receiving antigen-specific T-cell help?

An antigen through its BCR

What major processes are supported by CD40–CD40L signaling?

B-cell activation, antibody class switching, germinal center responses, and affinity maturation.

Which class of antigens typically induces T-dependent B-cell responses?

Protein antigens

Why is a second signal important for B-cell activation?

It helps ensure that antigen recognition leads to an appropriate immune response rather than inappropriate activation

What types of molecules can B cells recognize besides proteins?

Lipids, carbohydrates, and nucleic acids.

Why can't all B-cell responses depend exclusively on conventional helper T-cell recognition of antigens?

B cells recognize many nonprotein antigens, while conventional CD4+ helper T cells recognize peptides presented on MHC class II.

Which immunoglobulin isotypes are expressed by the mature naïve B cells?

IgM and IgD

Which immunoglobulin isotype is expressed by the immature naïve B cells?

IgM

What initiates the humoral immune response?

Recognition of microbial antigen by a naïve B cell.

What provides additional stimuli for B-cell activation?

Helper T cells or other activating signals

What happens during B-cell proliferation?

Activated B cells multiply, producing additional antigen-specific B cells

What is B-cell differentiation?

The development of activated B cells into specialized cells, including antibody-secreting plasma cells and memory B cells.

What is the primary function of plasma cells?

Antibody secretion

Which antibody is shown being secreted by the initial plasma cells?

IgM

What is isotype switching?

A change in the antibody class produced by a B cell, such as switching from IgM to IgG.

What is affinity maturation?

The process by which an antibody response develops antibodies with increased antigen-binding affinity

• EX: Production of high-affinity IgG antibodies

What is the function of memory B cells?

To support a faster and stronger response upon subsequent exposure to the same antigen

What are the four major outcomes shown after B-cell activation and differentiation?

Antibody secretion, isotype switching, affinity maturation, and memory B-cell formation.

• THESE CAN ONLY HAPPEN WITH THE HELP OF T CELLS!!!

Which B-cell subset is primarily associated with T-dependent antibody responses?

Follicular B cells

Which antibody classes are products of T-dependent responses?

IgG, IgA, and IgE.

What are the principal outcomes of T-dependent B-cell activation?

• Isotype-switching

• high-affinity antibodies

• Memory B cells

• Long-lived plasma cells.

Which B-cell subsets are associated with T-independent responses?

B-1 cells and marginal zone B cells.

What type of antigen activates B-1 and marginal zone B cells?

Polysaccharide antigen

What additional signals can contribute to T-independent B-cell activation?

Complement proteins and microbial products

Which antibody class predominates in the T-independent responses?

IgM

What is the typical affinity of antibodies produced during T-independent response?

Relatively low affinity.

What type of plasma cells are primarily generated in the T-independent response?

Short-lived plasma cells

How do T-dependent and T-independent responses differ in antibody class switching?

T-dependent responses efficiently undergo class switching, while the illustrated T-independent response produces mainly IgM.

What is one major advantage of T-independent B-cell activation?

It allows a speedy antibody response without waiting for helper T-cell assistance.

How does T-independent activation increase the diversity of adaptive immune responses?

It permits antibody responses against nonprotein antigens such as microbial polysaccharides.

What would the 2nd immune response to a polysaccharide antigen look like?

It would look like the 1st response!

• Why? T cells cannot recognize polysaccharides, so they can’t help produce memory cells to fight the 2nd infection

What class of antibodies will you see within the 1st week of a primary antibody response?

IgM

• For secondary responses, we mainly see isotype switching antigens

What is the usual lag period after immunization during a primary response?

Approximately 5–10 days.

What is the usual lag period after immunization during a secondary response?

Approximately 1–3 days

How does the peak antibody response compare between primary and secondary responses?

The primary peak is smaller; the secondary peak is larger.

What is the usual relative abundance of IgM and IgG during a primary response?

IgM is usually greater than IgG

What antibody isotype change characterizes many secondary responses?

A relative increase in IgG.

• IgA or IgE may increase during secondary responses under certain conditions (Heavy-chain isotype switching)

What is the typical average antibody affinity during a primary response?

Lower and more variable

What is the typical average antibody affinity during a secondary response?

Higher, as a result of affinity maturation

What initiates antigen receptor-mediated signal transduction in B lymphocytes?

Antigen-induced cross-linking of membrane immunoglobulin (mIg)

Which proteins associate with membrane immunoglobulin to transmit intracellular BCR signals?

Igα and Igβ.

Which kinase is recruited to phosphorylated ITAMs and propagates (multiplies) BCR signaling?

Syk

Which Src-family kinases are shown participating in early BCR signaling?

Fyn, Lyn, and Blk.

Which phospholipase is activated downstream of BCR signaling?

Phospholipase Cγ2 (PLCγ2).

What two major second-messenger pathways result from PLCγ2 activation?

The inositol trisphosphate (IP3)/calcium pathway and the diacylglycerol (DAG) pathway.

What is the major effect of inositol trisphosphate in the pathway shown?

Increased cytosolic Ca²⁺ concentration.

• Activation of calcium-dependent enzymes

Which transcription factor is activated downstream of calcium-dependent signaling?

NFAT

Which signaling molecule activates protein kinase C (PKC)?

Diacylglycerol (DAG).

Which transcription factor is activated downstream of the PKC pathway?

NF-κB

Which small GTP-binding proteins are shown in the BCR signaling pathway?

Ras and Rac

What molecular exchange activates Ras and Rac?

Exchange of GDP for GTP

Which downstream kinases are shown after Ras and Rac activation?

ERK and JNK

Which transcription factor is associated with the ERK/JNK signaling pathway?

AP-1

What change promotes increased B-cell survival and proliferation?

Increased expression of proteins that promote cell survival and cell cycling.

• Increased B Cell proliferation

What changes enable activated B cells to interact more effectively with helper T cells?

Antigen presentation and increased B7 expression

What is the functional consequence of increased cytokine receptor expression?

Greater responsiveness to cytokines.

What is the consequence of increased CCR7 expression on activated B cells?

Migration from the B-cell follicle toward the T-cell zone.

What is the consequence of the generation of plasma cells from activated B cells?

Antibody secretion

Where do B cells initially encounter antigen in the diagram?

The B-cell zone

Where do antigen-activated B cells and helper T cells initially interact?

Near the boundary between the T-cell and B-cell zones.

What two major pathways can follow the initial T–B cell interaction?

An extrafollicular response and a germinal center reaction

What is an extrafollicular focus?

A site outside the follicular germinal center where activated B cells differentiate into antibody-secreting cells.

• B cells go back into the follicular zone to proliferate!

What type of plasma cells are in the extrafollicular focus?

Short-lived plasma cells

Which helper T-cell population is associated with the extrafollicular response?

Extrafollicular helper T cells

Which specialized helper T-cell subset participates in the germinal center reaction?

Follicular helper T cells (Tfh cells)

What are the major products of the germinal center reaction shown?

Germinal center B cells differentiate into memory B cells and long-lived plasma cells.

How do extrafollicular and germinal center responses differ in plasma-cell longevity?

The extrafollicular response primarily generates short-lived plasma cells, while germinal center responses generate long-lived plasma cells.

How do rare antigen-specific B cells and helper T cells find each other?

Antigen activation changes their migration patterns, allowing them to move toward shared regions within lymphoid organs.

• EX: Cytokine secretion!

How does a helper T cell determine which B cell to assist?

It recognizes a specific peptide–MHC class II complex presented by that B cell.

What is the second step after antigen recognition by B cells to helper T cells?

Receptor-mediated endocytosis of the antigen.

What is receptor-mediated endocytosis?

Internalization of a bound antigen into the cell through a receptor-dependent process.

• Breaks down the pathogen inside the cell

What happens to the internalized protein antigen?

It is processed into peptide fragments and presented on MHC Class II molecules

Which T-cell subset recognizes antigen presented by B cells on MHC class II?

CD4+ helper T cells

What is the complete sequence of antigen presentation by B cells?

Native antigen recognition → receptor-mediated endocytosis → processing → MHC II peptide presentation → CD4+ T-cell recognition.

What two types of helper T-cell signals are shown activating B cells?

CD40L-mediated contact and secreted cytokines.

Where do helper T-cell cytokines bind?

Cytokine receptors on the B cell

What happens after CD40 engagement and cytokine signaling?

B-cell activation, proliferation, and differentiation.

What is the role of cytokines in T-dependent antibody responses?

They promote B-cell growth and differentiation and influence antibody isotype selection.

What is the Gram-stain classification of Haemophilus influenzae?

Gram-negative

What is the cellular shape of Haemophilus influenzae?

Coccobacillus