B2 W1 - Adaptive Immunity

Question 1

What are the two main branches of the immune system, and how do they interact?

Answer 1

The immune system comprises the innate and adaptive branches. The innate system often triggers and shapes the adaptive response, while the adaptive system can also provide feedback to enhance innate immunity.

Question 2

How do the innate and adaptive immune systems differ in terms of evolutionary history and speed of response?

Answer 2

The innate system is evolutionarily older and provides a rapid, non-specific response. The adaptive systemis more recent, providing a slower but highly specific and targeted response.

Question 3

What are the two main effector mechanisms found in both the innate and adaptive immune responses?

Answer 3

Both immune branches employ cell-mediated and humoral mechanisms. Cell-mediated immunity involves direct cell-to-cell interactions, while humoral immunity is mediated by soluble molecules.

Question 4

What is the role of cell surface receptors in immune responses, and in which branch are they particularly important?

Answer 4

Cell surface receptors are crucial for recognizing and responding to specific threats. They are particularly vital for the adaptive immune response, where highly specific receptors on B and T cells recognize distinct antigens.

Question 5

What are the two main types of lymphocytes involved in the adaptive immune response, and what key ability do they share?

Answer 5

The two main types of lymphocytes are B lymphocytes (B cells) and T lymphocytes (T cells). Both B cells and T cells possess receptors on their surface that can recognise and bind to specific antigens.

Question 6

Where do T cells mature, and how does this relate to their name?

Answer 6

T cells migrate to and mature in the thymus, which is why they are called T cells. This process ensures they develop the ability to recognise specific antigens.

Question 7

Where do mature lymphocytes reside and what triggers them to mount an immune response?

Answer 7

Mature T and B cells reside in peripheral lymphoid organs. When they encounter their specific cognate antigen, they become activated and initiate an immune response.

Question 8

How do B cells and cytotoxic T cells differ in their methods of combating pathogens?

Answer 8

B cells, when activated by an antigen, differentiate into plasma cells that produce and secrete antibodies. These antibodies target pathogens in body fluids. Cytotoxic T cells directly kill infected or cancerous cells by releasing cytotoxic molecules.

Question 9

What is the role of helper T cells in the adaptive immune response?

Answer 9

Helper T cells secrete signalling molecules called cytokines that stimulate and coordinate other immune cells, including B cells, cytotoxic T cells and macrophages. This “help” is often crucial for the full activation and effectiveness of other immune cells.

Question 10

What is the function of regulatory T cells in the immune system?

Answer 10

Regulatory T cells (T-regs) have a suppressive or downregulatory effect on immune responses. This helps to prevent excessive immune activation and maintain immune system balance, protecting against autoimmunity.

Question 11

Besides B and T cells, what other cell type plays a crucial role in activating the adaptive immune response?

Answer 11

Antigen presenting cells (APCs), such as dendritic cells and macrophages, engulf foreign material, break it down, and present fragments (antigens) on their surface using MHC molecules to activate T cells.

Question 12

What are memory cells and how do they contribute to long-term immunity?

Answer 12

Memory cells are long-lived B or T lymphocytes that persist in the body after an initial immune response. Upon subsequent encounters with the same antigen, memory cells enable a faster and more robust response, providing long-term immunity against specific pathogens.

Question 13

How does the adaptive immune system achieve the ability to potentially detect any antigen?

Answer 13

The adaptive immune system generates a vast diversity of B cells, each with a unique immunoglobulin receptor on its surface. This enormous repertoire of receptors, theoretically, allows for the recognition of any antigen that the body might encounter.

Question 14

How does the body regulate the immune response to prevent harmful reactions against itself (autoimmunity) and enhance responses against genuine threats?

Answer 14

The body controls the immune response by regulating the populations of B and T cells. Cells with receptors that recognise “self” antigens are eliminated to prevent autoimmunity. In contrast, cells with receptors that bind to foreign antigens are stimulated to proliferate and mount a targeted immune response.

Question 15

What is meant by humoral immunity, and what is its primary effector molecule?

Answer 15

Humoral immunity refers to the immune response mediated by molecules in extracellular fluids, primarily antibodies (also known as immunoglobulins).

Question 16

How does the B cell receptor relate to the antibodies produced by activated B cells?

Answer 16

The B cell receptor is a membrane-bound form of the antibody. Upon activation, the B cell differentiates into a plasma cell and secretes a soluble form of the same antibody that was initially present on its surface.

Question 17

How does the activation of a B cell lead to changes in its morphology and function?

Answer 17

When an antigen binds to the B cell receptor, it triggers a signalling cascade within the cell. This leads to changes in gene expression, resulting in the development of extensive endoplasmic reticulum needed for producing and secreting large amounts of soluble antibodies. The activated B cell transforms into a plasma cell, essentially becoming an “antibody factory.”

Question 18

What is clonal selection, and how does it ensure a targeted and effective humoral immune response?

Answer 18

Clonal selection is the process where a B cell that encounters and binds its cognate antigen is selectively stimulated to proliferate. This results in the production of a large clone of identical B cells, all producing the same antibody specific for the triggering antigen, thereby amplifying the immune response against that specific threat.

Question 19

What are the main functions of antibodies once they are secreted into the body fluids?

Answer 19

Antibodies can neutralise pathogens by blocking their ability to bind to host cells, opsonise pathogens to enhance their phagocytosis by macrophages and other phagocytes, and activate the complement system, a cascade of proteins that can directly kill pathogens or enhance other immune mechanisms.

Question 20

Define ‘epitope’.

Answer 20

An epitope is a specific region or structural feature on an antigen that is recognized by an antibody. Large antigens can have multiple epitopes, each potentially recognized by a different antibody.

Question 21

Explain what is meant by antibody affinity, and how it can vary.

Answer 21

Antibody affinity refers to the strength of binding between an antibody and its specific epitope. Different antibodies can have varying affinities for the same antigen, influenced by factors like shape complementarity, charge interactions and hydrophobic interactions. Higher affinity generally translates to more effective neutralisation or opsonisation of the antigen.

Question 22

How do polysaccharides differ from proteins in terms of their epitopes, and what implications does this have for antibody binding?

Answer 22

Polysaccharides often have repetitive epitopes, meaning the same structural feature is present in multiple copies. This allows for multiple identical antibodies to bind to the polysaccharide, creating a dense coating of antibodies. Proteins, in contrast, typically have more diverse epitopes, with different antibodies binding to distinct regions.

Question 23

Explain the concept of immunological memory and its importance in protecting against subsequent infections.

Answer 23

Immunological memory results from the generation of long-lived memory B cells and T cells during the initial immune response. If the same antigen is encountered again, these memory cells enable a much faster and stronger secondary response, often preventing or significantly reducing the severity of disease. This is the basis of vaccination, where exposure to a harmless form of an antigen primes the immune system to respond effectively to the actual pathogen in the future.

Question 24

Briefly describe the structure of a typical antibody molecule.

Answer 24

Antibodies, or immunoglobulins, typically have a Y-shaped structure composed of four polypeptide chains: two identical heavy chains and two identical light chains. The tips of the “Y” arms form the variable regions, responsible for antigen binding, while the stem of the “Y” constitutes the constant region, involved in effector functions like complement activation.

Question 25

What is antibody class switching, and why does it occur during an immune response?

Answer 25

Antibody class switching is a process where an activated B cell changes the class of antibody it produces, for example, from IgM to IgG. Different antibody classes have different effector functions and distributions in the body, allowing for a tailored response as the infection progresses. This switching involves changes in the DNA encoding the antibody heavy chain.

Question 26

What is unique about the DNA changes involved in antibody production and class switching?

Answer 26

To generate antibody diversity and enable class switching, the immune system employs DNA deletions and rearrangements. These processes are unusual in human cells, as they permanently alter the genetic code. However, they are crucial for creating the vast array of antibodies needed for effective immunity and tailoring the immune response over time.

Question 27

What is the central effector mechanism of cell-mediated immunity, and what cell type is primarily responsible?

Answer 27

Cell-mediated immunity involves the direct killing of infected or abnormal cells by cytotoxic T cells (also known as CD8+ T cells).

Question 28

How do cytotoxic T cells recognize and target infected cells?

Answer 28

Cytotoxic T cells recognize foreign antigens, such as viral peptides, that are presented on the surface of infected cells in conjunction with MHC class I molecules.

Question 29

Describe the structure of the T cell receptor (TCR) and its role in antigen recognition.

Answer 29

The TCR is a heterodimer, composed of an alpha and a beta chain, each with a variable and a constant domain. The variable domains form the antigen-binding site, providing specificity for a particular antigen.

Question 30

What is the function of CD3 in T cell activation?

Answer 30

CD3 is a complex of proteins associated with the TCR that plays a crucial role in signal transduction, transmitting the signal of antigen binding from the cell surface to the intracellular signalling pathways, leading to T cell activation.

Question 31

Why is the presentation of antigens on MHC molecules essential for T cell activation?

Answer 31

T cells cannot recognise free-floating antigens. They can only recognise and respond to antigens that are processed and presented on the cell surface in association with MHC molecules. This ensures that T cells target only cells displaying evidence of infection or abnormality.

Question 32

What are MHC molecules, and what are the two main classes involved in antigen presentation?

Answer 32

MHC molecules are cell surface proteins that present antigens to T cells. There are two main classes: MHC class I, found on most nucleated cells, and MHC class II, primarily found on professional antigen-presenting cells like dendritic cells and macrophages.

Question 33

How does the origin of the presented antigen differ between MHC class I and MHC class II molecules?

Answer 33

MHC class I molecules present antigens derived from intracellular proteins, typically viral peptides in infected cells. MHC class II molecules present antigens derived from material that has been ingested and processed by the antigen-presenting cell, such as fragments from bacteria or other pathogens.

Question 34

Which type of T cell recognizes antigens presented on MHC class I molecules, and what is the functional outcome of this interaction?

Answer 34

Cytotoxic T cells (CD8+ T cells) recognise antigens presented on MHC class I molecules. This interaction leads to the killing of the infected cell by the cytotoxic T cell, eliminating the source of infection.

Question 35

Which type of T cell recognizes antigens presented on MHC class II molecules, and what is the functional outcome of this interaction?

Answer 35

Helper T cells (CD4+ T cells) recognise antigens presented on MHC class II molecules. This interaction activates the helper T cell, which then releases cytokines to stimulate and coordinate other immune cells, including B cells, cytotoxic T cells, and macrophages.

Question 36

What are CD4 and CD8, and how do they contribute to T cell activation?

Answer 36

CD4 and CD8 are co-receptor molecules found on the surface of helper T cells and cytotoxic T cells, respectively. They bind to non-variable regions of MHC molecules, strengthening the interaction between the T cell and the antigen-presenting cell and enhancing T cell activation.

Question 37

Describe the mechanism by which cytotoxic T cells kill target cells.

Answer 37

Cytotoxic T cells release cytotoxic molecules, including perforin (which forms pores in the target cell membrane) and granzymes (proteases that enter through the pores and induce apoptosis). This targeted destruction eliminates infected cells without causing widespread damage to surrounding tissues.

Question 38

How does the MHC diversity within the human population impact disease susceptibility and transplant compatibility?

Answer 38

There are hundreds of different allelic variants of MHC genes in the human population. The specific MHC alleles an individual possesses can influence their ability to present antigens from certain pathogens, affecting their susceptibility to disease. MHC molecules also play a critical role in transplant rejection, as differences in MHC molecules between the donor and recipient can trigger an immune response against the transplanted tissue.