IMMU2011 - Weeks 4-5

Describe the structure of antibodies and how structure relates to function. [long response]

Antibodies are Y-shaped molecules made of two identical light chains and two identical heavy chains. Each chain has variable and constant domains. The variable regions form the antigen-binding site, enabling specificity, while the constant regions determine the antibody's isotype and effector function.

What gives antibodies the ability to bind diverse antigens?

their variable regions are generated by V(D)J recombination and junctional diversity, creating a vast repertoire of unique antigen-binding sites

What is the role of the antibody hinge region?

it provides flexibility, allowing antigen-binding sites to adjust to different epitope distances and orientations

What are the five isotypes of antibodies, and what do their constant regions determine?

IgM, IgG, IgA, IgE, IgD. The constant region determines their effector functions such as complement activation, opsonisation, or mucosal immunity.

Give details of how antibodies work. [long response]

Antibodies neutralize pathogens, opsonize microbes for phagocytosis, mediate ADCC, activate complement, and help clear parasites. These functions rely on the Fc region engaging Fc receptors or complement proteins.

Which antibody isotypes are involved in neutralisation?

IgG, IgM, IgA

How does IgA contribute to mucosal immunity?

IgA is transported across epithelial cells into mucosal secretions, where it neutralizes microbes, preventing their entry

What is opsonisation?

antibodies coat a pathogen, marking it for uptake by phagocytes via Fc receptors

How do antibodies assist in killing infected cells?

via ADCC, IgG-coated infected cells are recognized by NK cells through FcγRIII, triggering granule release and cell death

How do antibodies aid in the clearance of helminths?

IgE binds parasites; eosinophils recognize IgE through FcεRI and release toxic granules to kill the worms.

What is antibody-mediated mast cell activation? [long response]

IgE binds FcεRI on mast cells and basophils. Cross-linking by antigen triggers release of inflammatory mediators, causing allergic responses and protection against toxins.

Which isotypes activate the classical complement pathway?

IgG1 and IgG3

Explain how B cells develop. [long response]

B cells develop in the bone marrow, where they rearrange their antigen receptor genes to create a BCR. This involves V(D)J recombination and junctional diversity. They undergo selection to eliminate non-functional or self-reactive BCRs. Mature B cells express IgM and IgD and circulate to secondary lymphoid organs.

What is V(D)J recombination?

a process where variable (V), diversity (D), and joining (J) gene segments are randomly recombined to generate unique antigen receptor variable regions

What enzyme mediates V(D)J recombination?

VDJ recombinase, made of RAG-1 and RAG-2 proteins

What is junctional diversity?

the addition of random nucleotides at V-D and D-J junctions, increasing receptor diversity exponentially

What is positive selection in B cell development?

ensures the BCR is functional by checking if it can be translated into a complete receptor

What is negative selection in B cell development?

eliminates or edits B cells that bind strongly to self-antigens, preventing autoimmunity

Where is B cell development completed?

in the spleen, where follicular B cells mature and express both IgM and IgD

Describe the steps of a T-dependent B cell response to antigen. [long response]

1. Antigen encounter: B cell binds unprocessed antigen via its BCR in the follicle. 2. T cell help: CD4+ T cells recognize the same antigen (as processed peptide via MHC II) in the T cell zone. 3. Migration: B and T cells move toward each other and meet at the follicular border. 4. Activation: B cell presents antigen to T cell. CD40-CD40L interaction and cytokines from the T cell fully activate the B cell. 5. Extrafollicular response: Some B cells become short-lived plasma cells and secrete low-affinity antibodies. 6. Germinal center reaction: B cells undergo proliferation, somatic hypermutation, affinity maturation, and class switching with help from T follicular helper (Tfh) cells. 7. Outcome: Differentiation into long-lived plasma cells or memory B cells that circulate or reside in the bone marrow.

What is somatic hypermutation?

a process where mutations are introduced into the variable region of the BCR gene to increase affinity for the antigen

What is affinity maturation?

selection of B cells with higher-affinity receptors following somatic hypermutation

What is isotype switching?

T cell-mediated change in the antibody isotype produced, altering effector function without changing antigen specificity

How do B cells recognise antigens?

B cells recognise native antigens in their full form, including proteins, lipids, carbohydrates, and nucleic acids, without the need for processing

How do T cells recognise antigens?

T cells only recognise processed peptide antigens presented on MHC molecules; the antigen must be cell-associated

What is the key difference in antigen recognition between B and T cells?

B cells recognise unprocessed, soluble antigens, whereas T cells recognise processed peptide antigens on MHC molecules

What happens to dendritic cells (DCs) after encountering a pathogen?

DCs recognise PAMPs through PRRs, become activated, process antigens, and migrate to lymph nodes to present antigens to T cells

What are the four steps of T cell activation?

1) Antigen recognition 2) Co-receptor engagement (CD4/CD8) 3) Adhesion molecule interaction (LFA-1 - on T cells binding to ICAM-1 - on APCs) 4) Co-stimulatory signal.

What does the co-receptor CD4 bind to?

CD4 binds to the β2 domain of MHC class II molecules

What does the co-receptor CD8 bind to?

CD8 binds to the α3 domain of MHC class I molecules.

What is the two-signal hypothesis for T cell activation?

Signal 1 is TCR recognition of peptide-MHC, Signal 2 is co-stimulation via CD28 on T cells binding CD80/86 on APCs.

What is the immunological synapse?

the contact interface between a T cell and an APC where antigen recognition and signal exchange occurs

Why is co-stimulation important for T cells?

without co-stimulation, T cells may become anergic or tolerant, failing to activate even if they recognise antigen

What are the inhibitory receptors that regulate T cell activation?

CTLA-4 and PD-1; they bind to B7 and PD-L1/PD-L2 respectively to terminate T cell responses

What is cross-presentation?

the process by which DCs present extracellular antigens on MHC class I molecules to activate CD8+ T cells

What are the two main pathways of cross-presentation?

cytosolic (proteasome-dependent) and vacuolar (lysosome-dependent, TAP-independent)

Why is cross-presentation important?

it enables activation of CD8+ T cells against pathogens that do not directly infect DCs

What structural changes occur in activated B cells?

expanded ER, Golgi, and cytoplasmic volume to support antibody production

What structural changes occur in activated T cells?

increased mitochondria, ribosomes, and cytoplasmic volume; CD8+ cells develop cytotoxic granules

What signalling molecules are involved in TCR signal transduction?

CD3 complex (γ, δ, ε) and ζ chain containing ITAMs

Which transcription factors are activated upon TCR stimulation?

NFAT, AP-1, and NFκB, which promote IL-2 gene transcription

What is the role of IL-2 in T cell activation?

IL-2 promotes T cell proliferation and differentiation into effector cells

What is the composition of the B cell receptor (BCR)?

two identical heavy and two light chains forming a Y-shaped structure with variable regions for antigen binding

What are the two signals required for B cell activation?

Signal 1: Antigen binding to BCR; Signal 2: Co-stimulation via CD40/CD40L (T-dependent) or cross-linking (T-independent)

What are the major CD4⁺ T helper cell subsets?

Th1 and Th2

What cytokines do Th1 cells produce?

IFN-γ, IL-2

What cytokines do Th2 cells produce?

IL-4, IL-5, IL-13

What is the role of IFN-γ in host defence?

activates macrophages (M1), promotes antigen presentation, enhances killing of intracellular pathogens

What is the role of IL-4 in host defence?

stimulates B cell class switching to IgE, drives M2 macrophage activation, and promotes response to extracellular pathogens

What are the master regulators of Th1 and Th2 cells?

Th1: T-bet; Th2: GATA-3

Which innate cytokines promote Th1 differentiation?

IL-12, through activation of STAT4

Which innate cytokines promote Th2 differentiation?

IL-4, through activation of STAT6

How do Th1 and Th2 responses affect leprosy outcomes?

Th1 response (tuberculoid) controls infection; Th2 response (lepromatous) leads to disease progression

What are the functions of Th1 cells?

enhance macrophage microbial killing, stimulate cytotoxic T cell responses, support IgG opsonisation

What are the functions of Th2 cells?

promote humoral immunity, help clear helminths, support IgE production, tissue repair via M2 macrophages

How do CD4+ T cells help CD8+ T cells?

provide cytokines (e.g., IL-2) and enhance APC licensing to activate cytotoxic T cells

How do CD4+ T cells help B cells?

through CD40-CD40L interaction and cytokines like IL-4 and IL-21, promoting class switching and memory formation

What determines the differentiation of naive CD4+ T cells into specific subsets?

the cytokine environment and transcription factors such as T-bet or GATA-3

How does innate immunity shape CD4+ T cell differentiation?

pattern recognition by innate cells triggers cytokine production that directs Th subset differentiation

What do STAT proteins do in T cell differentiation?

transduce cytokine signals from IL-12 (via STAT4 for Th1) and IL-4 (via STAT6 for Th2) to activate master transcription factors

What is the importance of the Th1/Th2 paradigm?

it explains how distinct T cell responses drive different immune outcomes in infections, allergies, and autoimmune diseases

What happens when GATA-3 is over-expressed in Th1 cells?

they begin to produce Th2 cytokines, shifting to a Th2 phenotype

What immune responses are linked to Th1 cells?

cell-mediated immunity, especially against intracellular bacteria and viruses

What immune responses are linked to Th2 cells?

humoral immunity, particularly effective against helminths and involved in allergy