Lecture 06: T-Cell Activation and B cell Activation [SEQ 2]

MHC class I molecules are expressed on all nucleated cells in the body. They present fragments of intracellular proteins, including viral or bacterial proteins, to Tc cells. When a cell is infected, it processes foreign proteins through the proteasome and delivers them to the ER, where they bind to MHC class I molecules. The MHC I-peptide complexes are then transported to the cell surface. Tc cells, with their TCRs, recognize these MHC I-peptide complexes, initiating the activation of the Tc cell. This leads to the release of cytotoxic granules, containing perforin and granzymes, which induce apoptosis in the infected cell.

MHC class I are expressed on all nucleated cells, while MHC class II are expressed on antigen-presenting cells (APCs), such as macrophages, dendritic cells, and B cells. MHC I presents peptides derived from intracellular proteins to cytotoxic T cells (CD8+ T cells), while MHC II presents peptides derived from extracellular proteins to helper T cells (CD4+ T cells).

MHC I molecules are involved in presenting antigens from within a cell, such as viral proteins, to CD8+ T cells. MHC II molecules present antigens from outside the cell, like bacterial proteins, to CD4+ T cells. This allows the immune system to target both intracellular and extracellular pathogens.

Helper T cells (Th cells) play a crucial role in coordinating the immune response. They activate other immune cells, including B cells and macrophages, leading to a more robust and effective response. They secrete cytokines, such as IFN-gamma and TNF-alpha, that stimulate the proliferation and activation of Tc cells, promoting the killing of infected cells. Th cells also stimulate B cells to differentiate into plasma cells, which produce antibodies against specific antigens. Additionally, Th cells activate macrophages to kill engulfed microbes.

<p>When a naive T cell encounters its specific antigen presented by an APC, such as a dendritic cell (DC), it becomes activated. This activation triggers a process called clonal expansion, where the activated T cell rapidly divides and proliferates, producing many identical copies of itself.</p> <p>These daughter cells differentiate into two main types: effector cells and memory cells. Effector cells, such as cytotoxic T cells (CD8+) and helper T cells (CD4+), are specialized to carry out specific immune functions. Effector T cells leave the lymphoid tissue and travel to the site of infection, where they directly target infected cells or secrete cytokines to activate other immune cells.</p> <p>Memory cells are long-lived lymphocytes that remain in circulation, providing immunological memory against the specific antigen they encountered. They are primed to respond quickly and efficiently upon subsequent encounters with the same antigen, ensuring a faster and more effective immune response in the future.</p> Signup and view all the answers

<p>HIV infects and destroys CD4+ T cells, specifically helper T cells, which are crucial for coordinating the immune response. This depletion of CD4+ T cells impairs the ability of the immune system to fight off infections, making individuals with HIV susceptible to opportunistic infections and malignancies. AIDS is the final stage of HIV infection, characterized by a severely compromised immune system. As HIV progresses, CD4+ T cell counts decline, and the individual becomes increasingly vulnerable to a wide range of opportunistic infections. The loss of immune function makes it difficult to control these infections, leading to complications and, eventually, death.</p> Signup and view all the answers

<p>B cells become activated when they encounter their specific antigen. This activation can occur in two ways: independently, through direct antigen binding or through T cell-dependent activation.</p> <p>In T cell-dependent activation, a B cell presents the antigen it has bound to a helper T cell (Th cell) via MHC II molecules. This interaction stimulates the Th cell to release cytokines, which activate the B cell.</p> <p>Activated B cells differentiate into plasma cells that secrete antibodies, proteins that bind to specific antigens and neutralize pathogens.</p> <p>Memory B cells are long-lived B cells that are generated during the primary immune response. These cells are programmed to remember the specific antigen they encountered and can rapidly differentiate into plasma cells upon re-exposure to the same antigen, leading to a faster and more efficient secondary immune response.</p> Signup and view all the answers

<p>The immune response to a primary infection is typically slower and less effective than the response to a secondary infection. During the primary infection, naive T and B cells need to be activated, and the immune system has to build up a response against the pathogen. This process takes time. Memory cells, generated during the primary response, play a critical role in the faster and more robust response to a secondary infection.</p> <p>When an individual encounters the same pathogen again, the memory cells, which are specific to that antigen, quickly recognize and respond to the antigen. This leads to rapid differentiation of memory B cells into antibody-producing plasma cells and the activation of memory T cells, providing a more rapid and effective response.</p> Signup and view all the answers

<p>Antigen-presenting cells (APCs) are the primary link between the innate and adaptive immune systems. APCs, such as dendritic cells (DCs), macrophages, and B cells, engulf pathogens and process their antigens. They then present these antigens to T cells, initiating the adaptive immune response.</p> <p>DCs, for example, are specialized phagocytic cells that reside in tissues. When they encounter a pathogen, they engulf it, process its antigens and migrate to lymph nodes, where they encounter and activate naive T cells. Macrophages, also involved in phagocytosis, present antigens to helper T cells, which activate other immune cells to fight the infection. B cells can also present antigens to helper T cells, further amplifying the immune response.</p> Signup and view all the answers

<p>B cells in the lymph node encounter soluble antigen, which binds to their IgD receptors. This activates the B cell, causing it to internalize the antigen, process it, and present it via MHCII to T helper (Th) cells. If a Th cell with a matching TCR recognizes the antigen, the B cell is further activated. This activates the B cell to divide and differentiate into plasma cells, undergoing clonal selection to produce large numbers of antibody-producing plasma cells, which release antibodies specific to the antigen.</p> Signup and view all the answers

<p>Lymph node macrophages are phagocytic cells that engulf and destroy large antigens, such as viruses, while lymph fluid carries soluble, smaller antigens to the lymph nodes. This allows for the sorting of antigens based on their size, ensuring efficient presentation to the appropriate immune cells. Macrophages engulf large antigens, preventing their spread, while soluble antigens in lymph fluid can activate B cells, initiating antibody production.</p> Signup and view all the answers

<p>T cells recognize antigen presented on MHC molecules by antigen-presenting cells (APCs), primarily dendritic cells (DCs). They do not bind directly to free antigen. B cells, however, can bind directly to soluble antigens via their surface IgD receptors. Both T cells and B cells require a signal from another immune cell for full activation. T cells require activation from a DC, while B cells need activation from a Th cell that recognizes the same antigen it has processed and presented through MHCII. Both T and B cell activation leads to clonal proliferation, generating numerous effector cells that are specific to the antigen.</p> Signup and view all the answers

<p>The 4-7 days required for a B cell to fully activate and proliferate in the lymph node allows for the generation of a large number of plasma cells (up to 5000). These plasma cells produce a large quantity of antibodies specific to the antigen, ensuring a robust and sustained response to the invading pathogen. The delay in B cell activation provides time for the immune system to evaluate and respond to the specific threat, minimizing unnecessary activation and potential inflammation.</p> Signup and view all the answers

<p>Clonal selection ensures that only B cells that specifically recognize the invading antigen proliferate and differentiate into antibody-producing plasma cells. Each B cell has a unique IgD receptor that binds to a specific antigen. When an antigen enters a lymph node, only the B cells with the matching IgD receptors are activated and proliferate. This process ensures that the immune response is targeted toward the specific pathogen, preventing unnecessary activation of other B cells and maximizing the effectiveness of the antibody production.</p> Signup and view all the answers

White blood cells that mature in the thymus and play a central role in immune response.

Also known as CD8 or Tc cells, they kill cancerous or infected cells directly.

CD4 or Th cells that activate other immune cells via cytokine release.

Major histocompatibility complex molecules that present antigens to T-cells.

Presents antigens from all nucleated cells to cytotoxic T cells, signaling infection.

Found on antigen-presenting cells like dendritic cells, presenting microbial peptides to helper T cells.

The process where antigen-presenting cells display antigens to activate T-cells.

Rapid proliferation of activated T cells in response to a pathogen, leading to effector cells.

T cells recognize antigens and become active; they check dendritic cells in lymph nodes.

B cells bind to antigens and get activated by helper T cells after detection.

Lymph nodes filter lymph fluid for antigens; site for T and B cell activation.

Activated B cells can become plasma cells, producing large amounts of antibodies.

When activated, B cells proliferate and differentiate into plasma cells that create antibodies.