The adaptive immune response to viral infections is a highly specific and targeted defense mechanism that involves the activation of immune cells and the production of antibodies to eliminate the invading virus. This response is characterized by immunological memory, allowing the immune system to mount a faster and more effective defense upon subsequent exposures to the same virus.

1. Antigen Recognition:
   • Dendritic Cells: Antigen-presenting cells (APCs) such as dendritic cells capture viral antigens and present them on their surfaces.
   • Helper T Cells (CD4+): Recognize the viral antigens presented by APCs and become activated.
2. Helper T Cell Activation:
   • Activated helper T cells release cytokines that stimulate various immune responses.
   • Interleukin-2 (IL-2): Promotes the activation and proliferation of T cells.
   • Interferon-gamma (IFN-γ): Enhances the antiviral activity of other immune cells.
3. Cytotoxic T Cell Activation:
   • Cytotoxic T Cells (CD8+): Activated by helper T cells, cytotoxic T cells recognize and directly kill virus-infected cells.
   • Perforin and Granzymes: Released by cytotoxic T cells to induce apoptosis (cell death) in infected cells.
4. B Cell Activation:
   • Recognition of Antigens: B cells directly recognize viral antigens or receive help from activated helper T cells.
   • B Cell Differentiation: Activated B cells differentiate into plasma cells, which produce antibodies specific to the viral antigens.
5. Antibody Production (Humoral Immunity):
   • Antibodies (Immunoglobulins):
     • Released into the bloodstream, mucosa, and other bodily fluids.
     • Bind to viral particles, neutralizing them and preventing them from infecting host cells.
     • Mark viruses for destruction by phagocytic cells.
     • Activate the complement system, enhancing the immune response.
6. Memory T and B Cells:
   • Memory T Cells: Long-lived T cells that “remember” the viral antigens. Provide rapid and enhanced responses upon re-exposure.
   • Memory B Cells: Long-lived B cells that can quickly differentiate into plasma cells upon re-exposure, producing antibodies.
7. Immunization (Vaccination):
   • Principle: Vaccines stimulate the adaptive immune system without causing the disease, inducing immunological memory.
   • Memory Response: If vaccinated individuals encounter the actual virus, their immune system mounts a faster and more effective response.
8. Regulatory T Cells:
   • Function: Suppress excessive immune responses to prevent autoimmune reactions and excessive inflammation.