Influenza is a communicable disease caused by a virus. Explain how the human body responds to an influenza infection, from the point at which the virus enters the body to the destruction of the pathogen.

1. The virus first encounters the non‑specific barrier of the respiratory tract – mucus, cilia and secreted antibodies – which traps and removes some particles.
2. If the virus penetrates, innate immune cells (macrophages, dendritic cells) recognise viral antigens on its surface.
3. These cells present the antigens to lymphocytes, which activate B‑cells to produce antibodies that are complementary to the viral antigens.
4. The antibodies bind to the virus, neutralising it and causing agglutination, which prevents attachment to host cells.
5. Antibody‑coated viruses are recognised by phagocytes, which engulf and digest them; memory B‑cells remain for a faster response if the virus re‑enters.