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Questions and Answers

What is primarily mediated by B cells in adaptive immunity?

Humoral immunity through antibody production (correct)

Cell-mediated immunity

Memory cell generation

Production of cytokines

What do T cells require for the recognition of foreign proteins?

Direct interaction with the pathogen

Interaction with B cells only

Binding to antibodies

Presentation of processed peptides by MHC (correct)

Naive lymphocytes refer to which of the following?

Lymphocytes that have proliferated

Mature lymphocytes that have encountered antigen

Lymphocytes that have produced antibodies

Mature T or B cells that have never encountered foreign antigen (correct)

What is the role of Helper T cells in adaptive immunity?

To coordinate the immune response by interacting with other immune cells Signup and view all the answers

Which pathway is involved in presenting intracellular pathogen proteins to T cells?

MHC Class I pathway Signup and view all the answers

What happens to B cells in the bone marrow if they do not successfully complete receptor editing?

They undergo apoptosis. Signup and view all the answers

What characteristic distinguishes memory cells in adaptive immunity?

They respond more quickly upon re-exposure to the same antigen Signup and view all the answers

Which T cell type is specifically recognized by MHC Class I molecules?

Cytotoxic T cells Signup and view all the answers

Which of the following is NOT one of the three signals required for T cell activation?

Recognition of MHC Class I. Signup and view all the answers

How do B cells recognize antigens?

Directly via their B cell receptors (BCR) Signup and view all the answers

What role does CD40L on the activated Helper T cell play in B cell activation?

It provides co-stimulation. Signup and view all the answers

Where does the interaction between lymphocytes and antigens primarily take place in the body?

Lymph nodes. Signup and view all the answers

Which of the following statements about lymphoid tissues is correct?

Lymph nodes are encapsulated organs distributed throughout the body. Signup and view all the answers

What mechanism allows B cells to recognize antigens?

BCR - Membrane-bound antibody. Signup and view all the answers

How are antigens delivered to lymph nodes from tissues?

Via lymphatics. Signup and view all the answers

What type of antigens lead to full B cell activation through the interaction with Helper T cells?

T-dependent protein antigens. Signup and view all the answers

What is the primary consequence of CD8 T cell interaction with MHC class I molecules?

Death of the presenting cell Signup and view all the answers

Which molecules are recognized by CD4 T cells?

MHC class II Signup and view all the answers

Which of the following cells are classified as professional antigen-presenting cells?

B cells Signup and view all the answers

What is the structural composition of a BCR/antibody?

2 identical heavy chains and 2 identical light chains held together by disulphide bonds Signup and view all the answers

How is the diversity of TCRs and BCRs generated?

By recombination of gene segments Signup and view all the answers

What occurs during the process of negative selection for T cells in the thymus?

T cells that recognize self-peptides undergo apoptosis Signup and view all the answers

What is the primary role of dendritic cells in the immune system?

Processing and presenting antigens Signup and view all the answers

What polymorphic genes are associated with MHC Class I molecules in humans?

HLA-A, HLA-B, and HLA-C Signup and view all the answers

What is the primary function of the spleen in the immune system?

To filter and eliminate blood-borne pathogens Signup and view all the answers

Which class of antibody is the first one produced during an immune response?

IgM Signup and view all the answers

What do B cells require to undergo isotype switching and produce different classes of antibodies?

Cytokine signals Signup and view all the answers

What characteristic of antibodies allows Natural Killer (NK) cells to recognize and eliminate antibody-coated cells?

Fc receptor expression Signup and view all the answers

Which of the following roles do antibodies NOT directly participate in?

Antigen presentation Signup and view all the answers

What is the main effect of T regulatory cells (Treg) in the immune system?

Inhibiting immune responses to maintain self-tolerance Signup and view all the answers

During an immune response, what typically happens to most lymphocytes generated through clonal expansion?

They undergo apoptosis Signup and view all the answers

Which type of T cell is primarily involved in direct killing of infected cells?

Cytotoxic T cells Signup and view all the answers

Study Notes

Adaptive Immunology

Adaptive immunity is a feature of the immune system that is characterized by the ability to recognize specific antigens.
Key features of adaptive immunity include antigen presentation, lymphocyte activation, B cell effector function, T cell effector function, and memory.
A microbe is engulfed by a phagocyte and encased in a phagosome.

Lesson Plan

Features of adaptive immunity
Antigen presentation
Lymphocyte activation
B cell effector function
T cell effector function
Memory

Adaptive Immunity

Innate immunity occurs within hours after infection, whereas adaptive immunity takes days.
B lymphocytes and T lymphocytes make up the adaptive immune system.

Phases of Adaptive Immunity

Antigen is a molecule specifically recognised by lymphocytes or antibodies.
Naive lymphocytes (B and T cells) haven't encountered foreign antigens.
Binding to specific antigens leads to activation, proliferation, and differentiation of B and T cells. This takes time.

Functions of Adaptive Immunity

B cells mediate humoral immunity by producing antibodies.
Helper T cells orchestrate an effective immune response by interacting with other immune cells.
Cytotoxic T cells kill infected or malignant host cells.

Memory in the Adaptive Immunity

Following infection resolution, some cells persist (memory cells), responding faster the next time, preventing or lessening the infection's severity.
The primary response involves initial encounter with an antigen; the secondary response is a quicker, more potent reaction due to memory cells.

TCR and BCR

Surface immunoglobulin (B cell receptor) and T cell receptor (TCR) allow cells to recognize an antigen.
BCR can directly recognize an antigen
TCR cannot directly recognize an antigen; proteins must be processed into peptides before antigen presentation via MHC to the T cell.

Recognition of Antigen

B cells express a B-cell receptor (BCR) and T cells express a T-cell receptor (TCR).
BCR can directly recognize antigens
TCR cannot directly recognize antigens; proteins must be processed before presentation in MHC.

Antigen Presentation

T cells cannot directly recognize foreign proteins.
Proteins must be processed into peptides, bound to self molecules known as MHC (Human Leukocyte Antigen).
MHC Class I pathway is for intracellular pathogens (where the foreign protein is made by a host cell).
MHC Class II pathway is for extracellular pathogens.

Antigen Presentation Class I and Class II

MHC Class I molecules bind to peptides from inside cells, used for cells infected by intracellular microbes.
MHC Class II molecules bind to peptides from outside the cell, and are used for cells infected by extracellular microbes.

MHC Class I Pathway

Foreign proteins produced inside cells are presented on MHC Class I molecules.
Cytotoxic T cells recognize the MHC Class I-protein complex, leading to the infected cell's death.
The marker CD8 acts as a co-receptor for MHC Class I.

MHC Class II Pathway

Foreign proteins taken up by presenting cells are presented on MHC Class II molecules.
Helper T cells recognize the MHC Class II-protein complex leading to the helper T cell's activation.
The marker CD4 act as a co-receptor for MHC Class II.

MHC Pathways

Proteins made inside cells (e.g. virally infected), are digested into peptides in proteasomes for MHC class I presentation.
Proteins taken up from outside (e.g., phagocytosis), are digested into peptides by lysosomal enzymes for MHC class II presentation.

MHC Class I

Expressed on all nucleated cells.
The most polymorphic genes in the human genome (variants of HLA-A, HLA-B, HLA-C, differing in peptide-binding).

MHC Class II

Expressed by immune cells (dendritic cells, macrophages, and B cells).
Six main MHC class II genes in humans (HLA-DPA1, HLA-DPB1, ..., HLA-DRB1).
Very polymorphic genes.

Dendritic Cells (DCs)

Play a central role in antigen processing and presentation.
Immature DCs capture antigens in the periphery and become mature DCs in lymph nodes; presenting antigens to helper T cells.
DCs are professional antigen-presenting cells (APCs), along with macrophages and B cells.

Diversity of TCR and BCR

Variable domains of both BCR and TCR are generated via recombination of gene segments, resulting in a large number of different receptors from a limited number of gene segments.

BCR/Antibody Structure

BCR/antibody molecules are composed of two identical heavy chains and two identical light chains, held together by disulphide bonds.
Each chain contains constant and variable regions. The variable regions are on the antigen-binding site.
Light chain: one V domain, one C domain.
Heavy chain: one V domain, 3-4 C domains.

TCR Structure

TCR consists of two chains (α and β), held together by disulphide bridges.
Each chain has one constant and one variable region. The variable regions are on the antigen-binding site.

Negative Selection

Random generation of BCR and TCR from gene segments means some may recognise self antigens.
Immature T cells in the thymus recognising self-peptides on MHC undergo apoptosis (negative selection).
B cells in bone marrow may undergo receptor editing or apoptosis (negative selection).

Activation of Adaptive Cells

T helper cells are activated via three signals:
Antigen recognition by TCR
Co-stimulation by presenting cell
Cytokine signaling
Recognition of the antigen by TCR without costimulation and/or cytokine signals will not result in T helper cell activation. APC (antigen-presenting) cells are activated with PRR (pattern recognition receptor) engagement; APC costimulation without antigen recognition does not lead to activation.

Signals for T cells Activation

APC activation is necessary for T cell activation. Activated APCs produce co-stimulatory molecules and cytokines.
Antigen recognition in the absence of signal (2 & 3) does not lead to T cell activation.

Signals for B cell Activation

B cell receptor (BCR) recognizes antigen.
Co-stimulation from helper T cell.
Cytokines.

B cell activation

Antigen recognition by BCR leads to internalization and processing of the antigen.
Processing of the antigen with MHC Class II presentation.
Recognition of the antigen-MHC complex by TCR on activated helper T cells (CD4+ T cells).
This allows co-stimulation and full B cell activation.
This process only occurs with T cell-dependent protein antigens

B cell- T cell interactions

Co-stimulation occurs between CD40L on the activated helper T cell and CD40 on the B cell.
Cytokines from the helper T cell can influence the type of antibody produced.

Lymphocyte Trafficking

Antigens from body tissues are delivered to lymph nodes via lymphatics.
Antigens in the blood are delivered to the spleen.
Primary lymphoid tissues (bone marrow and thymus): generate and mature immune cells.
Secondary lymphoid tissues (lymph nodes and spleen): allow lymphocytes to interact with antigens and become activated.

Lymph Node

Encapsulated organs in the body, filtering antigens from the periphery.
Ag is moved through lymphatics by antigen-presenting cells (APCs).
Follicles allow T and B cells to encounter their antigens and proliferate, leading to mature effector B cells.

The Spleen

A highly vascular organ that filters blood and eliminates pathogens (blood-borne pathogens)
Follicles (like those in lymph nodes): antigen-encountering areas where proliferation leads to fully mature T and B cells.

Effector Functions of Adaptive Lymphocytes

B cell function involves antigen recognition, plasma cell production, and antibody production, including neutralization of microbes, phagocytosis, and complement activation.

Antibody Isotypes

5 classes/isotypes of antibodies (IgG, IgA, IgE, IgM, IgD)
Different constant regions (Fc fragments) affect the biological activity (e.g., complement fixation).
IgM is the first antibody produced.

Antibody Isotype Switching

Depending on signals, B cells can change antibody isotype (e.g. IgM to IgG, IgE, IgA).
Different isotypes mediate different effector functions (e.g., complement activation).

Antibody Isotype Differences

Antibody forms (monomer or dimer), percentage in serum.
Locations in the body/ where they function.
Specific antibody functions (e.g., protection from external openings, reacting to infection).
Transferability (via colostrum, breast milk, placenta).

Role of Antibodies

Antibodies are produced by the adaptive immune system (many effector functions are dependent on innate immunity - such as phagocytosis and complement activation).

Antibodies: Neutralisation

Antibodies block the entry of microbes or toxins.
Prevents microbes or toxins from interacting with cells.

Antibodies: Opsonisation

Opsonization is the process of coating a pathogen with antibodies (IgG).
Phagocytes (Fc receptors) engulf the coated pathogen, promoting phagocytosis.

Antibodies: ADCC

Antibody-dependent cell-mediated cytotoxicity (ADCC)
Antibody-coated cells (e.g. some microbes or infected cells) are recognized by natural killer (NK) cells that have Fc receptors, leading to elimination of the targeted cell.

T cell Function

Helper T cells (CD4+) produce cytokines that activate macrophages, which activate T and B cells.
Cytotoxic T cells (CTL, CD8+) kill infected cells.

Cytotoxic T Cells

Kill infected cells
Cytotoxic T cell mechanism similar to NK cells.
Accessory protein (CD8) aids in recognition of Class I MHC molecule on the infected cell via TCR.
Perforin and granzymes induce infected cell apoptosis.

T Helper Cells

Distinct T helper cell subsets (Th1, Th2, Th17, Treg) depending on the cytokine environment during T helper activation.
Important for maintaining self-tolerance and inhibiting immune responses.

T Helper Cell Subsets

Th1 cells activate macrophages.
Th2 cells activate eosinophils and mast cells.
Th17 cells recruit neutrophils.
Treg cells inhibit immune responses.

Memory

Most lymphocytes generated during an immune response eventually die, but some become memory B and T cells.
Memory cells can quickly be reactivated compared to naïve B and T cells.

Second Response

Secondary response is more potent and faster than the primary response due to memory cells.
Fewer antigens are required for a robust response to be initiated and maintained.

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