L15 - Lymphocyte development: B-cell development

What are the Two Universal Rules of Immunology?

1. Receptor–ligand interactions are required for recognition

2. Responses are often mediated by cytokines and chemokines

How does Antigen Recognition differ between Innate and Adaptive Immunity?

• Innate: conserved motifs via PRRs, generally shared across cells

• Adaptive: diverse epitopes via clonally distributed BCRs and TCRs

How does the Timing of Innate vs. Adaptive Immunity differ?

• Innate responses are immediate

• Adaptive responses are delayed

What are the Main Lymphocyte Types and Their Receptors?

• B cells express B-cell receptors (BCRs)

• T cells express T-cell receptors (TCRs)

What are the Four Main Hematopoietic Lineages?

1. Lymphocytic

2. Granulocytic

3. Megakaryocytic

4. Erythrocytic

What Structural Features do BCRs and TCRs share?

1. Antigen-binding sites

2. Variable regions

3. Constant regions

4. Transmembrane regions

How do BCRs differ from TCRs in Antigen Binding and Secretion?

• BCRs have two equivalent binding sites and can be secreted as antibodies

• TCRs have one binding site and remain cell-bound

What is the Basic Structure of an Immunoglobulin?

Based on: Peptide structure

• Two identical light chains

• Two identical heavy chains

• Hinge region

Based on: Function

• Variable and region

• Constant region

What are the Functions of the Variable and Constant Regions?

• Variable region: antigen specificity

• Constant region: antibody function

QUESTION: How Many Epitopes can One Immunoglobulin bind, and are They Identical?

ANSWER → Two epitopes simultaneously

• Both binding sites recognize the same epitope

What are the Fab, Fc, and Hinge Regions?

• Fab binds antigen

• Fc interacts with Fc receptors on other cells

• Hinge provides flexibility

QUESTION: What can a Macrophage with an Fc Receptor bind?

ANSWER → The constant (Fc) region of an immunoglobulin

What are the Five Immunoglobulin Isotypes?

Their classes are determined by the heavy-chain type

1. IgG

2. IgM

3. IgD

4. IgA

5. IgE

How do IgA and IgM differ in their Multimeric Structures?

• IgA can form dimers

• IgM can form pentamers

  • Increasing their cross-linking potential

What is the Main Characteristic of IgM?

First antibody isotype to appear and disappear after infection

• Often associated with recent exposure

What is the Main Characteristic of IgG?

Most abundant and long-lasting immunoglobulin in blood

• Commonly monitored in vaccine titers

What are the Main Functions of IgE and IgA?

IgE: allergies and parasite responses

IgA: mucosal protection, including the intestinal lumen

What is the Difference between an Antigen and an Epitope?

• Antigen: whole molecule

• Epitope: specific portion recognized by an antigen receptor

What Types of Molecules can B Cells recognize as Antigens?

1. Proteins

2. Lipids

3. Carbohydrates

4. Nucleic acids such as DNA

What is the Difference between Linear and Conformational Epitopes?

• Linear: consecutive amino acids

• Conformational: amino acids brought together by protein folding or assembly

What happens to Conformational Epitopes during Denaturation?

They may be lost when tertiary or quaternary structure is disrupted

What are Hypervariable Regions, and Which is Most Important?

Highly variable amino acid regions within the variable region of an immunoglobulin that interact with antigens and determine binding specificity

• Also known as complementarity-determining regions: CDR1, CDR2, and CDR3

  • CDR3 is especially important for specificity

QUESTION: What Two Mechanisms initially generate BCR Diversity?

1. Random recombination of gene segments

   • Different V, D, and J gene segments are randomly combined to create unique BCRs

2. Addition of random nucleotides at joining sites

   • Extra nucleotides are added at the junctions between gene segments, creating even more diversity

Why is Somatic Recombination Necessary for BCR Diversity?

It generates millions of receptor specificities from a limited number of protein-coding genes

What is Combinatorial Diversity?

Creation of different receptor sequences by mixing and matching available gene segments

How does Random Nucleotide Addition increase BCR Diversity?

Adding nucleotides at gene-segment junctions creates additional unique sequences

Where and When does Initial BCR Diversity develop?

In the bone marrow during B-cell development, BEFORE antigen exposure

What is the Principle of Clonal BCR Specificity?

1. Each B-cell clone has one antigen-binding specificity

2. Different clones can recognize different epitopes

Which Gene Segments form the Heavy and Light Chains?

• Heavy chain: V, D, and J segments

• Light chain: V and J segments

Why can Heavy Chains generate Greater Junctional Diversity than Light Chains?

• Heavy chains have three segment types and two junctions

• Light chains have two types and one junction

What is V(D)J Recombination?

DNA-level rearrangement of V, D, and J gene segments that generates diverse antigen receptors

QUESTION: Why is DNA-Level BCR Rearrangement Important?

Daughter cells inherit the rearranged receptor genes and maintain the same antigen specificity

QUESTION: Which Molecules transmit Signals from the BCR into the Cell?

ANSWER → Igα & Igβ

• Which relay antigen recognition into intracellular signaling pathways and ultimately the nucleus

QUESTION: How are Membrane-Bound and Secreted Immunoglobulins generated?

ANSWER → Differential mRNA processing (alternative splicing) produces membrane or secretory heavy-chain tails

QUESTION: Does Alternative Splicing change the Antigen Specificity of an Immunoglobulin?

ANSWER → No

• Membrane-bound and secreted forms recognize the same epitope

QUESTION: How do you think the two tails differ in their amino acid composition?

A hydrophobic segment in its membrane tail anchors it to the cell membrane

What is the Difference between V(D)J Recombination and Alternative Splicing?

• V(D)J: permanent DNA rearrangement for diversity

• Splicing: RNA processing for membrane vs. secreted forms

What Two Requirements must Developing B Cells meet before Maturation?

1. They must express a functional antigen receptor

2. Avoid inappropriate self-reactivity

What is the Goal of B-Cell Selection in the Bone Marrow?

To produce functional, non-self-reactive B cells ready to respond to foreign antigens

PRACTICE QUESTION: Unlike in innate immunity, B-cell receptors:

a) Are entirely germline encoded

b) Recognize the exact same repeating structures

c) Cannot directly bind an epitope

d) Uniquely recognize a single epitope

ANSWER → d) Uniquely recognize a single epitope

PRACTICE QUESTION: The ability to bind to many different types of epitopes is:

a) Due to recombination events during B-cell development

b) After the epitope is encountered

c) When random nucleotides are incorporated

d) T-cell dependent

e) A and C

ANSWER → e) A and C

• a) Due to recombination events during B-cell development

• c) When random nucleotides are incorporated

PRACTICE QUESTION: B-cells with antibodies reactive to an epitope, can lose this reactivity during class switch recombination (making a different isotype)

a) True

b) False

ANSWER → False

TABLE 1. Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Recognition	Broadly conserved motifs (PAMPs/DAMPs)	Highly diverse epitopes
Receptors	Pattern recognition receptors (PRRs)	BCRs and TCRs
Receptor distribution	Generally similar across cells of the same type	Clonal; different lymphocytes have different receptors
Receptor diversity	Hundreds	Millions (~10 million)
Response timing	Immediate	Delayed
Generation of diversity	Receptors encoded in inherited genes	Somatic gene rearrangement

Remember: Innate immunity recognizes shared patterns, while adaptive immunity recognizes highly specific epitopes.

TABLE 2. BCR vs. TCR vs. Secreted Antibody

Feature	BCR	TCR	Secreted Antibody
Found on	B cells	T cells	Soluble form
Antigen-binding sites	2	1	2
Variable region	Yes	Yes	Yes
Constant region	Yes	Yes	Yes
Membrane-bound	Yes	Yes	No
Secreted form	Yes, as antibody	No	Already secreted
Main role	Antigen recognition	Antigen recognition	Antigen binding and immune functions

Key concept: 

• BCRs and secreted antibodies can recognize the same epitope.

• TCRs remain membrane-bound.

TABLE 3. Immunoglobulin Structure and Function

Component	Structure / Function
Heavy chains	2 identical chains
Light chains	2 identical chains
Variable region	Determines antigen specificity
Constant region	Determines antibody function
Fab region	Fragment antigen-binding; binds epitopes
Fc region	Fragment crystallizable; binds Fc receptors on other cells
Hinge region	Provides flexibility to antigen-binding arms
Antigen-binding sites	2 identical binding sites recognizing the same epitope

Remember: 

• Variable = specificity

• Constant = function

TABLE 4. Immunoglobulin Isotypes

Isotype	Main Characteristic	Clinical Relevance
IgM	First to appear and disappear after infection; can form pentamers	Often indicates recent exposure
IgG	Most abundant and long-lasting in blood	Vaccine titers and longer-term antibody responses
IgE	Associated with allergies and parasites	Hypersensitivity and parasite responses
IgA	Secreted at mucosal barriers; can form dimers	Mucosal immunity, especially the gut
IgD	Fifth isotype; not emphasized	Know it exists

TABLE 5. Antigen Recognition: Linear vs. Conformational Epitopes

Feature	Linear Epitope	Conformational Epitope
Other name	Continuous	Discontinuous
Amino acids	Consecutive in sequence	Brought together by folding or assembly
Depends on 3D structure?	No	Yes
Effect of denaturation	May remain recognizable	May be lost
Structural basis	Primary sequence	Tertiary or quaternary structure

Remember:

• Antigen = entire molecule

• Epitope = specific portion recognized

TABLE 6A. BCR Mechanisms of Diversity

Mechanism	How It Works	Result
Combinatorial diversity	Random mixing of gene segments	Different receptor sequences
Random nucleotide addition	Adds nucleotides at joining sites	Additional sequence diversity
Heavy/light chain pairing	Different heavy and light chains combine	Further increases diversity
V(D)J recombination	Rearranges receptor genes at DNA level	Stable specificity inherited by daughter cells

TABLE 6B. Heavy vs. Light Chain Recombination

Feature	Heavy Chain	Light Chain
Gene segments	V, D, J	V, J
Number of segment types	3	2
Joining sites	2	1
Junctional diversity	Greater	Less
Rearrangement	DNA level	DNA level

Important: BCR diversity is initially generated in the bone marrow before antigen exposure.

TABLE 7A. BCR Generation vs. Antibody Secretion vs. B-Cell Development: DNA Recombination vs. RNA Splicing

Feature	V(D)J Recombination	Alternative Splicing
Level	DNA	RNA
Purpose	Generate receptor diversity	Produce membrane-bound or secreted Ig
Permanent DNA change?	Yes	No
Inherited by daughter cells?	Yes	No
Changes antigen specificity?	Yes, during initial receptor generation	No
Key result	Unique BCR specificity	Different forms with the same specificity

TABLE 7B. BCR Generation vs. Antibody Secretion vs. B-Cell Development: B-Cell Development and Selection

Stage / Concept	Key Event
Bone marrow development	BCR genes undergo rearrangement
Functional receptor selection	Developing B cell must produce a functional receptor
Self-reactivity selection	Inappropriately self-reactive cells are selected against
Central tolerance	Selection against self-reactivity in bone marrow
Peripheral tolerance	Additional tolerance mechanisms outside bone marrow, including spleen
Final result	Mature, functional, non-self-reactive B cells