The secondary immune response is the body's faster, stronger, and more prolonged reaction upon re-exposure to an antigen that has been encountered before. This enhanced response is due to the presence of memory cells generated during the primary immune response. Here's how it involves B and T lymphocytes: B Lymphocytes (B cells): 1. Memory B Cells: During the primary response, some activated B cells differentiate into long-lived memory B cells. These cells retain the "memory" of the specific antigen. 2. Rapid Activation: Upon re-exposure to the same antigen, memory B cells are quickly activated. They do not require as much co-stimulation as naive B cells. 3. Proliferation and Differentiation: Activated memory B cells rapidly proliferate and differentiate into a large number of plasma cells. 4. Antibody Production: These plasma cells produce a much higher quantity and affinity of antibodies compared to the primary response. The antibodies are primarily of the IgG class, which are more effective at neutralizing pathogens and marking them for destruction. T Lymphocytes (T cells): 1. Memory T Cells: Similarly, some activated T cells (both helper T cells and cytotoxic T cells) differentiate into long-lived memory T cells. 2. Rapid Activation: When memory T cells encounter their specific antigen presented by antigen-presenting cells (APCs), they are rapidly activated. 3. Helper T Cells (Memory CD4+ T cells): These cells quickly proliferate and release cytokines that enhance the activation and proliferation of memory B cells and cytotoxic T cells. 4. Cytotoxic T Cells (Memory CD8+ T cells): These cells rapidly proliferate and become effective killers, directly destroying infected host cells that display the specific antigen. Key differences from the primary response: Lag Phase: Shorter lag phase (time before response is detected). Peak Response: Higher peak antibody levels and a greater number of effector T cells. Duration: Response is sustained for a longer period. Antibody Affinity: Antibodies produced have a higher affinity for the antigen. Predominant Antibody: IgG is the main antibody, whereas IgM is dominant in the primary response.