The adaptive immune response, a sophisticated defense mechanism, relies on the activation and differentiation of B lymphocytes, or B cells. These processes enable our bodies to mount a tailored response against specific pathogens such as bacteria, free virus particles, toxins, and parasites.

When naive B cells encounter a specific antigen that can bind to the B cell receptor (BCR) on their surface, they undergo sensitization to respond to the antigen's presence. Sensitization begins with antigen recognition, causing the B cell to engulf the antigen through receptor-mediated endocytosis. Once inside the B cell, the antigen is broken down into smaller peptide fragments, which combine with MHC II molecules. This antigen-MHC II complex is then transported to the B cell surface, where it is displayed, completing the sensitization process.

After sensitization, the B cell acts as an antigen-presenting cell for T cells. The binding of the MHC II-complex antigen to the T cell receptor (TCR) on a Helper T cell releases cytokines, which serve as co-stimulatory signals to activate the B cell. Upon activation, the B cell undergoes clonal expansion, rapidly proliferating to generate identical B cells capable of binding to the same antigen. Some activated cells differentiate into effector plasma cells, producing antibodies. The antibodies can neutralize the antigen, preventing it from interacting with host cells or mark it for destruction by phagocytes. Simultaneously, other activated B cells differentiate into long-lived memory B cells that provide immunological memory upon subsequent encounters with the same antigen.