Lecture 06: T-Cell Activation and B-cell activation [SEQ 1]

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Questions and Answers

Explain how a B-cell recognizes and processes an antigen, highlighting the key stages and the cellular interactions involved.

B-cells recognize soluble antigens present in the lymph fluid. They bind the antigen with their IgD receptors, internalize it through endocytosis, and process it into smaller peptides. These peptides are then displayed on the B-cell surface via MHC II molecules. This presentation allows the B-cell to interact with a T helper cell that possesses a complementary TCR to the presented antigen. This interaction is critical for full B-cell activation and antibody production.

Describe the time frame and key events involved in B-cell activation, starting from antigen detection to plasma cell differentiation.

Upon antigen detection, a B-cell remains within the lymph node. It then proceeds to process the antigen and present it on MHC II molecules, searching for a T helper cell with a matching TCR. Within approximately 48 hours, a successful interaction with a Th cell occurs, leading to B-cell activation. Over the following 4-7 days, the activated B-cell undergoes rapid proliferation, differentiating into plasma cells, which are highly specialized antibody-producing cells.

Compare and contrast the roles of T-cells and B-cells in antigen recognition and subsequent activation.

T-cells recognize antigen fragments presented on MHC molecules by antigen-presenting cells like dendritic cells. They require this interaction to be activated and differentiate into effector cells (Tc and Th cells). B-cells, on the other hand, directly recognize free antigens in lymph fluid using IgD receptors. While their initial activation involves antigen binding, they need further activation by Th cells to become fully functional antibody-producing plasma cells.

Why is the interaction between a B-cell and a Th cell crucial for optimal antibody production? Explain the role of each cell type in this process.

The interaction between a B-cell and a Th cell is vital for optimal antibody production because it ensures a coordinated and specific immune response. The B-cell presents the processed antigen on MHC II, and the Th cell, with a matching TCR, recognizes it. This interaction triggers a cascade of events, causing the B-cell to become fully activated and differentiate into plasma cells. The Th cell also provides essential signals, including cytokines, that further enhance B-cell proliferation and antibody production. Signup and view all the answers

Explain the significance of clonal selection in the context of both B-cell and T-cell activation.

Clonal selection is a critical process in both B-cell and T-cell activation. It ensures that only those cells with receptors specific for the invading antigen proliferate and differentiate into effector cells. In the case of B-cells, clonal selection leads to the generation of plasma cells that produce antibodies specifically targeting the antigen. For T-cells, clonal selection produces cytotoxic T-cells that can directly eliminate cells displaying the antigen, and helper T-cells that can support other immune cells. This process ensures a highly targeted and effective immune response. Signup and view all the answers

Explain the role of MHC I molecules in the immune response to intracellular pathogens, highlighting the key cells involved and the steps of this process.

MHC I molecules are expressed on all nucleated cells, presenting fragments of intracellular proteins, including those from viruses or bacteria, to cytotoxic T cells (CD8+). This process begins with the breakdown of foreign proteins by proteasomes within the infected cell. These peptides are then transported to the endoplasmic reticulum (ER) and loaded onto MHC I molecules. Upon recognition of the presented antigen by the TCR of a cytotoxic T cell, the T cell activates and releases cytotoxic granules that kill the infected cell, preventing further spread of the pathogen. Signup and view all the answers

How do helper T cells (Th cells) contribute to the activation of both cytotoxic T cells (Tc cells) and B cells, and what is the significance of this role in the overall immune response?

Helper T cells (Th cells) play a crucial role in orchestrating the immune response by activating both cytotoxic T cells (Tc cells) and B cells. They recognize antigens presented on MHC II molecules by antigen-presenting cells (APCs) like dendritic cells (DCs) and macrophages. Upon activation, Th cells secrete cytokines like IFN-γ and TNF-α, which directly stimulate Tc cells to proliferate and differentiate into effector cells capable of killing infected cells. Additionally, Th cells secrete cytokines like IL-4 and IL-5 that stimulate B cells to differentiate into antibody-producing plasma cells and memory cells. This coordinated activation of both Tc cells and B cells by Th cells allows for a robust and multifaceted immune response against pathogens, leading to their elimination from the body. Signup and view all the answers

Compare and contrast the mechanisms of antigen presentation by MHC I and MHC II molecules, highlighting the type of cells involved and the consequences of antigen recognition by each MHC type.

MHC I molecules are expressed on all nucleated cells and present peptides derived from intracellular proteins, including those from viruses or bacteria. This presentation is recognized by cytotoxic T cells (CD8+), leading to the destruction of the infected cell. MHC II molecules, on the other hand, are found on antigen-presenting cells (APCs) like dendritic cells and macrophages. They present peptides derived from extracellular pathogens that have been internalized and processed by these cells. This presentation is recognized by helper T cells (CD4+), which then activate other immune cells like B cells and macrophages to produce antibodies and eliminate pathogens. Signup and view all the answers

Describe the process of clonal expansion of T cells, emphasizing the role of antigen-presenting cells (APCs) and the different fates of activated T cells.

When a naive T cell encounters its specific antigen presented on an antigen-presenting cell (APC), it becomes activated. This activation triggers a process called clonal expansion, where the activated T cell rapidly divides, producing many copies of itself. Over a period of one to two days, the activated T cells differentiate into two main types: effector cells and memory cells. Effector T cells, including helper T cells (Th) and cytotoxic T cells (Tc), leave the lymphoid tissue and migrate to the site of infection to fight the pathogen. Memory T cells remain in the lymphoid tissue, providing long-term immunity by quickly responding to future encounters with the same antigen. Signup and view all the answers

What are the key functions of helper T cells in the immune response, and how does their activation contribute to the overall effectiveness of the immune system?

Helper T cells (Th cells) play a crucial role in orchestrating the immune response by activating other immune cells, including cytotoxic T cells (Tc cells) and B cells. They recognize antigens presented on MHC II molecules by antigen-presenting cells (APCs) like dendritic cells (DCs) and macrophages. Upon activation, Th cells release cytokines that stimulate Tc cells to proliferate and differentiate into effector cells that kill infected cells. Th cells also activate B cells to differentiate into antibody-producing plasma cells and memory cells. This coordinated activation of both Tc cells and B cells by Th cells ensures a multifaceted immune response that effectively eliminates pathogens. Signup and view all the answers

Explain how the activation of B cells differs from that of T cells, focusing on the role of antigen presentation and the specific functions of activated B cells.

Unlike T cells, which require antigen presentation on MHC molecules, B cells can be activated directly by free antigens in the lymph nodes or spleen. When a B cell encounters its specific antigen, it internalizes and processes the antigen, presenting fragments on its MHC II molecules. This antigen-MHC II complex then signals the B cell to activate and differentiate. However, for full activation and differentiation into antibody-secreting plasma cells, B cells often require interactions with activated helper T cells. Activated B cells differentiate into antibody-producing plasma cells, which secrete antibodies that can neutralize pathogens directly or mark them for destruction by phagocytes. Some activated B cells also become memory cells, providing long-term immunity by quickly responding to future encounters with the same antigen. Signup and view all the answers

Discuss the consequences of HIV infection on the function of the immune system, focusing on the specific cells targeted by the virus and the resulting immune deficiencies.

HIV, the virus that causes AIDS, primarily targets CD4+ T cells, also known as helper T cells. These cells play a crucial role in coordinating the immune response by activating both cytotoxic T cells and B cells. As HIV replicates and destroys CD4+ T cells, the immune system becomes progressively weakened. Without sufficient CD4+ T cells, the body is unable to effectively mount immune responses against infections and diseases. This immune deficiency makes individuals with AIDS highly susceptible to opportunistic infections and cancers, leading to a significant decline in health and ultimately death. Signup and view all the answers

Compare and contrast the roles of cytotoxic T cells (Tc cells) and macrophages in the immune response, highlighting their respective targets and mechanisms of action.

Both cytotoxic T cells (Tc cells) and macrophages play important roles in eliminating pathogens, but they target and act differently. Tc cells recognize and kill infected cells that present antigens on their MHC I molecules. They achieve this by releasing cytotoxic granules containing perforin and granzyme, which create pores in the target cell membrane, allowing for the entry of granzyme and triggering apoptosis, or programmed cell death. Macrophages, on the other hand, engulf and destroy pathogens or cellular debris through phagocytosis. They can also contribute to the immune response by presenting antigens on MHC II molecules to helper T cells, thus activating other immune cells. While Tc cells target infected cells directly, macrophages play a broader role in removing pathogens and initiating adaptive immune responses. Signup and view all the answers

Flashcards

Antigen recognition

T cells and B cells recognize and react to specific antigens for activation.

B cell activation process

B cells stay in lymph nodes, bind antigens with IgD, and present via MHC II.