Chapter 21 - Section 21.5 - Study Guide - Assess Your Learning Outcomes - Page 845: 2

When an immunocompetent B cell encounters a foreign antigen, it initiates a series of events that are crucial for the adaptive immune response. Here's a step-by-step overview of how this process unfolds: 1. **Antigen Recognition and Activation:** - Antigen Presenting Cells (APCs) such as dendritic cells, macrophages, and B cells phagocytose or engulf foreign antigens. - The engulfed antigens are broken down into smaller peptide fragments within the APCs. - MHC-II (Major Histocompatibility Complex Class II) proteins, located on the surface of APCs, bind to these antigenic peptide fragments. MHC-II proteins act as display platforms for antigens. 2. **Antigen Presentation:** - The MHC-II protein complex loaded with the antigenic peptide is transported to the surface of the APC. - The antigenic peptide-MHC-II complex is displayed on the cell surface, making it visible to T helper cells (CD4+ T cells). 3. **T Cell Activation:** - A specific CD4+ T helper cell with a complementary T cell receptor (TCR) recognizes the antigenic peptide-MHC-II complex on the APC. - This interaction between the TCR and the antigen-MHC-II complex, along with co-stimulatory signals (such as CD28 binding to B7 on the APC), activates the CD4+ T cell. 4. **B Cell Interaction:** - The activated CD4+ T cell releases cytokines like interleukins that help activate the B cell. - The cytokines, particularly IL-4 and IL-21, stimulate the B cell to undergo clonal expansion, differentiating into plasma cells (antibody-producing cells) and memory B cells. 5. **Clonal Expansion and Antibody Production:** - The B cell undergoes multiple rounds of division, creating a clone of identical B cells, all specific to the same foreign antigen. - These B cells differentiate into plasma cells, which are highly specialized for producing antibodies. - Antibodies are proteins that can specifically bind to the foreign antigen and neutralize it. 6. **Memory B Cell Formation:** - Some of the activated B cells differentiate into memory B cells instead of plasma cells. - Memory B cells are long-lived and "remember" the antigen they encountered. They provide a rapid and robust response if the same antigen is encountered in the future. In summary, the interaction between the immunocompetent B cell, MHC-II proteins, and CD4+ T cells plays a pivotal role in the adaptive immune response. The B cell's encounter with a foreign antigen triggers a cascade of events that lead to the production of antibodies, which are essential for neutralizing pathogens and providing immunity.

When an immunocompetent B cell encounters a foreign antigen, it initiates a series of events that are crucial for the adaptive immune response. Here's a step-by-step overview of how this process unfolds: 1. **Antigen Recognition and Activation:** - Antigen Presenting Cells (APCs) such as dendritic cells, macrophages, and B cells phagocytose or engulf foreign antigens. - The engulfed antigens are broken down into smaller peptide fragments within the APCs. - MHC-II (Major Histocompatibility Complex Class II) proteins, located on the surface of APCs, bind to these antigenic peptide fragments. MHC-II proteins act as display platforms for antigens. 2. **Antigen Presentation:** - The MHC-II protein complex loaded with the antigenic peptide is transported to the surface of the APC. - The antigenic peptide-MHC-II complex is displayed on the cell surface, making it visible to T helper cells (CD4+ T cells). 3. **T Cell Activation:** - A specific CD4+ T helper cell with a complementary T cell receptor (TCR) recognizes the antigenic peptide-MHC-II complex on the APC. - This interaction between the TCR and the antigen-MHC-II complex, along with co-stimulatory signals (such as CD28 binding to B7 on the APC), activates the CD4+ T cell. 4. **B Cell Interaction:** - The activated CD4+ T cell releases cytokines like interleukins that help activate the B cell. - The cytokines, particularly IL-4 and IL-21, stimulate the B cell to undergo clonal expansion, differentiating into plasma cells (antibody-producing cells) and memory B cells. 5. **Clonal Expansion and Antibody Production:** - The B cell undergoes multiple rounds of division, creating a clone of identical B cells, all specific to the same foreign antigen. - These B cells differentiate into plasma cells, which are highly specialized for producing antibodies. - Antibodies are proteins that can specifically bind to the foreign antigen and neutralize it. 6. **Memory B Cell Formation:** - Some of the activated B cells differentiate into memory B cells instead of plasma cells. - Memory B cells are long-lived and "remember" the antigen they encountered. They provide a rapid and robust response if the same antigen is encountered in the future. In summary, the interaction between the immunocompetent B cell, MHC-II proteins, and CD4+ T cells plays a pivotal role in the adaptive immune response. The B cell's encounter with a foreign antigen triggers a cascade of events that lead to the production of antibodies, which are essential for neutralizing pathogens and providing immunity.