B細胞の活性化と分化

Sensitization begins when a naive B cell encounters an antigen that binds to its B cell receptor (BCR) on the surface. The B cell then engulfs the antigen through receptor-mediated endocytosis, internalizing the antigen-BCR complex. Inside the cell, the antigen is processed into smaller peptide fragments and combined with MHC II molecules, which are transported to the B cell surface for display.

After sensitization, the B cell presents the antigen-MHC II complex to Helper T cells. When the Helper T cell's TCR binds to this complex, the Helper T cell releases cytokines as co-stimulatory signals that activate the B cell. This interaction is essential for triggering the B cell's proliferation and differentiation into effector and memory cells.

Upon activation by Helper T cell signals, the B cell undergoes clonal expansion, rapidly proliferating to generate many identical B cells capable of binding to the same antigen. This expansion amplifies the immune response by creating a large population of cells with identical antigen specificity, enabling a more effective defense against the pathogen.

Some activated B cells differentiate into effector plasma cells, which produce antibodies that neutralize antigens or mark them for destruction by phagocytes. Other activated B cells become long-lived memory B cells that provide immunological memory, enabling rapid recognition and response upon future encounters with the same antigen.

B cells mount tailored responses against specific pathogens including bacteria, free virus particles, toxins, and parasites. Through antibody production and antigen presentation, B cells enable the adaptive immune system to recognize and eliminate these diverse threats with precision and specificity.

Memory B cells generated during initial antigen exposure provide immunological memory that enables faster and stronger responses to subsequent encounters with the same antigen. This long-lived population of cells allows the immune system to recognize previously encountered pathogens more quickly, providing enhanced protection against reinfection.

During sensitization, the internalized antigen is broken down into smaller peptide fragments and combined with MHC II molecules to form an antigen-MHC II complex. This processed complex is transported to the B cell surface where it is displayed, allowing the B cell to function as an antigen-presenting cell for Helper T cells and enabling proper immune activation.