1. Describe the roles of T-cells in the immune response. How do T-cells contribute to immunity?
Answer:
T-cells are a vital part of the adaptive immune system, playing a key role in the cellular immune response. They originate from the bone marrow but mature in the thymus. T-cells are divided into several subtypes: helper T-cells, cytotoxic T-cells, and regulatory T-cells.
- Helper T-cells (CD4+ cells): They activate and regulate other immune cells by releasing cytokines. They help B-cells produce antibodies and activate cytotoxic T-cells and macrophages.
- Cytotoxic T-cells (CD8+ cells): These cells directly kill infected or cancerous cells by recognizing antigens on MHC I molecules on the surface of these cells. They induce apoptosis in the infected cells.
- Regulatory T-cells: These help maintain immune tolerance by suppressing the immune response to prevent autoimmune diseases.
2. What is the main function of B-cells in the immune system, and how do they contribute to the humoral immune response?
Answer:
B-cells are responsible for the humoral immune response, which involves the production of antibodies. When a B-cell encounters an antigen (usually a pathogen), it becomes activated. The activated B-cell differentiates into plasma cells, which secrete antibodies specific to the antigen. These antibodies bind to the antigen, neutralize it, and mark it for destruction by other immune cells. B-cells also form memory B-cells, which remain in the body to provide a quicker and stronger immune response if the same pathogen is encountered again.
3. Explain the process of T-cell activation. What signals are required for T-cell activation?
Answer:
T-cell activation involves a process where T-cells recognize and respond to antigens. The process requires two signals:
- Signal 1: The T-cell receptor (TCR) on the T-cell surface binds to an antigen presented by an antigen-presenting cell (APC) on Major Histocompatibility Complex (MHC) molecules. In the case of cytotoxic T-cells, the antigen is presented on MHC Class I molecules, while helper T-cells recognize antigens on MHC Class II molecules.
- Signal 2: Co-stimulatory signals provided by the APC (such as the interaction between CD28 on the T-cell and B7 molecules on the APC) are essential for full activation. Without this second signal, T-cells become anergic (inactive).
Once activated, T-cells proliferate and differentiate into effector T-cells that carry out the immune response.
4. How do B-cells become activated and differentiate into plasma cells?
Answer:
B-cell activation occurs when the B-cell receptor (BCR) binds to its specific antigen. This binding leads to internal signaling that activates the B-cell. However, full activation usually requires assistance from helper T-cells. Helper T-cells release cytokines that further stimulate B-cells. Once activated, the B-cell proliferates and differentiates into plasma cells, which are responsible for producing antibodies. Some B-cells also become memory B-cells, which persist long-term and provide immunity against future infections by the same pathogen.
5. Compare and contrast the roles of helper T-cells and cytotoxic T-cells in the immune response.
Answer:
- Helper T-cells (CD4+): These cells play a central role in coordinating the immune response. They do not directly kill pathogens but release cytokines that activate other immune cells. They help B-cells produce antibodies, stimulate macrophages, and activate cytotoxic T-cells.
- Cytotoxic T-cells (CD8+): These cells directly kill infected cells. They recognize and bind to infected cells displaying foreign antigens on their surface through MHC Class I molecules. Upon recognition, cytotoxic T-cells release perforins and granzymes that induce apoptosis (programmed cell death) in the infected cells.
While both T-cell types are involved in the adaptive immune response, helper T-cells primarily regulate and coordinate, while cytotoxic T-cells actively eliminate infected cells.
6. What is the role of memory cells (T-memory and B-memory) in immunity?
Answer:
Memory cells are long-lived immune cells that “remember” previously encountered pathogens.
- Memory T-cells: These are derived from activated helper T-cells and cytotoxic T-cells. They circulate in the body after the primary infection and can respond more rapidly if the same pathogen is encountered again.
- Memory B-cells: These are formed after B-cells are activated and differentiate into plasma cells. Memory B-cells can quickly differentiate into plasma cells upon re-exposure to the same pathogen, producing antibodies faster and in larger quantities than during the primary immune response.
Both types of memory cells contribute to the rapid and efficient immune response during subsequent exposures, forming the basis of immunological memory.
7. How do T-cells recognize infected cells, and what is their mechanism of action in eliminating these cells?
Answer:
T-cells recognize infected cells through the interaction of their T-cell receptors (TCRs) with antigens displayed on the surface of cells by Major Histocompatibility Complex (MHC) molecules. Cytotoxic T-cells specifically recognize antigens presented on MHC Class I molecules, which are found on nearly all cells in the body. Once the T-cell receptor binds to the antigen-MHC complex, the cytotoxic T-cell releases cytotoxic molecules like perforins, which create pores in the infected cell membrane, and granzymes, which trigger apoptosis. This leads to the destruction of infected or cancerous cells.
8. What are the differences between MHC Class I and MHC Class II molecules, and how do they relate to T-cell activation?
Answer:
MHC Class I molecules are present on almost all nucleated cells and present endogenous antigens (e.g., viral or tumor antigens) to cytotoxic T-cells (CD8+). These antigens are derived from within the cell.
MHC Class II molecules are expressed on specialized antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B-cells. They present exogenous antigens (e.g., from pathogens that have been phagocytized) to helper T-cells (CD4+).
The interaction between the T-cell receptor (TCR) and the antigen-MHC complex is essential for T-cell activation. The co-stimulatory signals ensure that the activation process is complete and that the immune response is appropriately targeted.
9. What is the significance of the interaction between helper T-cells and B-cells during an immune response?
Answer:
Helper T-cells and B-cells interact closely during an immune response to ensure an effective defense against pathogens. When B-cells encounter an antigen, they require activation signals from helper T-cells to fully respond. Helper T-cells secrete cytokines that enhance B-cell proliferation, differentiation into plasma cells, and antibody production. Additionally, helper T-cells help in the class switching of antibodies (e.g., from IgM to IgG), improving the effectiveness of the immune response. This interaction is critical for the humoral immune response and the production of antibodies that neutralize pathogens.
10. Describe the process of clonal selection and expansion in B-cells and its importance in immunity.
Answer:
Clonal selection refers to the process by which a specific B-cell is selected to proliferate and differentiate into plasma cells in response to an antigen. When a B-cell encounters its specific antigen, it binds to it via the B-cell receptor (BCR). This leads to activation, and the B-cell undergoes clonal expansion, producing many copies of itself. Some of these daughter cells become plasma cells that secrete large quantities of antibodies, while others become memory B-cells. Clonal selection ensures that only B-cells specific to the pathogen proliferate, allowing the immune system to mount a targeted response.
11. What are the functions of regulatory T-cells, and how do they maintain immune tolerance?
Answer:
Regulatory T-cells (Tregs) play an essential role in maintaining immune tolerance and preventing autoimmunity. They suppress the activity of other immune cells, particularly helper T-cells and cytotoxic T-cells, thereby ensuring that the immune response is not overly aggressive. Tregs help prevent the immune system from attacking the body’s own tissues (self-tolerance). They achieve this by producing anti-inflammatory cytokines like IL-10 and TGF-β, which dampen immune activation and ensure the immune response is only activated when necessary.
12. What is the role of the thymus in the development of T-cells?
Answer:
The thymus is a primary lymphoid organ where T-cells mature. T-cells originate in the bone marrow but must travel to the thymus for maturation. In the thymus, T-cells undergo selection processes:
- Positive selection: T-cells that can properly bind to MHC molecules are allowed to survive.
- Negative selection: T-cells that bind too strongly to self-antigens presented by MHC molecules undergo apoptosis to prevent autoimmune reactions.
The thymus ensures that only functional, self-tolerant T-cells are released into circulation, which are crucial for a properly functioning immune system.
13. Explain the concept of immune tolerance and how it is related to T-cells and B-cells.
Answer:
Immune tolerance refers to the immune system’s ability to distinguish between self and non-self, preventing immune responses against the body’s own tissues. T-cells and B-cells undergo a process of selection during their development to ensure that they do not react against self-antigens:
- Central tolerance: T-cells and B-cells that react strongly to self-antigens are eliminated during development in the thymus and bone marrow, respectively.
- Peripheral tolerance: Tregs and mechanisms such as anergy and deletion help maintain tolerance in peripheral tissues by suppressing potentially autoreactive cells that escape central tolerance.
Both T-cells and B-cells play a crucial role in ensuring that the immune system does not attack the body itself.
14. How do antibodies produced by B-cells neutralize pathogens?
Answer:
Antibodies (immunoglobulins) produced by B-cells play a key role in neutralizing pathogens. Once antibodies bind to an antigen, they can neutralize the pathogen in several ways:
- Neutralization: Antibodies bind to toxins or viruses, preventing them from interacting with host cells.
- Opsonization: Antibodies mark pathogens for phagocytosis by immune cells like macrophages.
- Activation of the complement system: Antibody-antigen complexes can activate the complement system, leading to the destruction of the pathogen.
These actions effectively limit the ability of pathogens to cause infection.
15. What is the role of antigen-presenting cells (APCs) in the activation of T-cells?
Answer:
Antigen-presenting cells (APCs), including dendritic cells, macrophages, and B-cells, play a critical role in initiating the T-cell-mediated immune response. APCs capture and process antigens, presenting them on their surface in association with MHC molecules. For T-cells to be activated, they must recognize and bind to the antigen-MHC complex via their TCRs. APCs also provide co-stimulatory signals (such as B7-CD28 interaction) that are required for full T-cell activation. Thus, APCs are essential for bridging the innate and adaptive immune systems.
16. What is the difference between the primary and secondary immune responses involving T-cells and B-cells?
Answer:
The primary immune response occurs when the body encounters a pathogen for the first time. It involves the activation of naive T-cells and B-cells. This response is slower and less intense as the immune system has not yet encountered the pathogen.
In contrast, the secondary immune response occurs upon re-exposure to the same pathogen. Memory T-cells and B-cells, which were formed during the primary response, recognize the pathogen more quickly and initiate a faster and more robust immune response. The secondary response is stronger because memory cells can rapidly differentiate into effector cells and produce antibodies.
17. Describe the process of class switching in B-cells and its importance in immunity.
Answer:
Class switching refers to the process where B-cells change the type of antibody they produce without altering their specificity for the antigen. Initially, B-cells produce IgM antibodies, but upon receiving signals from helper T-cells, they can switch to producing other classes of antibodies like IgG, IgA, or IgE. This process allows the immune response to adapt and provide more efficient protection. For example, IgG is more effective in neutralizing pathogens and activating the complement system, while IgA is crucial for mucosal immunity.
18. How do T-cells and B-cells work together in the immune response against viruses?
Answer:
T-cells and B-cells collaborate to provide a comprehensive immune response to viral infections.
- T-cells: Cytotoxic T-cells recognize and destroy infected cells displaying viral antigens on MHC Class I molecules, preventing the virus from replicating within the host. Helper T-cells activate B-cells and other immune cells, coordinating the immune response.
- B-cells: Activated B-cells produce antibodies that can neutralize free-floating viral particles, preventing them from entering healthy cells. These antibodies also mark viruses for destruction by phagocytic cells.
The coordinated action of both T-cells and B-cells provides a multifaceted defense against viral infections.
19. How does the concept of immune memory apply to T-cells and B-cells in long-term immunity?
Answer:
Immune memory is the ability of the immune system to remember past infections and respond more rapidly upon subsequent encounters with the same pathogen. Both T-cells and B-cells form memory cells during the primary immune response. Memory T-cells persist in the body and can quickly recognize and respond to the pathogen in future infections. Memory B-cells remain in circulation and can quickly differentiate into plasma cells to produce large amounts of antibodies when re-exposed to the same antigen. Immune memory is the basis of vaccination, where exposure to a weakened or inactivated pathogen leads to the formation of memory cells and protection against future infections.
20. Discuss the roles of T-cells and B-cells in autoimmune diseases.
Answer:
In autoimmune diseases, the immune system mistakenly targets the body’s own cells and tissues.
- T-cells: Regulatory T-cells usually prevent autoimmune reactions by suppressing autoreactive T-cells. However, in autoimmune diseases, this mechanism fails, leading to the activation of autoreactive T-cells that attack healthy tissues.
- B-cells: B-cells can produce antibodies against self-antigens, contributing to the development of autoimmune diseases like lupus or rheumatoid arthritis. These autoantibodies attack the body’s own cells, leading to inflammation and tissue damage.
In both cases, the immune system loses its ability to distinguish between self and non-self, causing harm to the individual’s own tissues.
These questions cover key concepts related to the roles of T-cells and B-cells in immunity, their differences, and their involvement in diseases.
