Host cells recognize a pathogen through viral sensing. The epithelial cells are the first line of defense, secreting lysozyme and lactoferrin. This, in turn, induces interferons that reduce or limit the virus's spread to other cells. Some of these interferons activate immune cells that start phagocytizing viruses, while other interferons released from dendritic cells stimulate T cells to secrete cytokines and chemokines. These interferons can impair the virus's ability to enter the cell, which prevents the virus from replicating. Still, other interferons degrade viral proteins. The production of these interferons also stimulates the adaptive immune response of macrophages and lymphocytes.
Neutrophils are the first cells to arrive. They can ingest the virions and destroy them. Next, macrophages will come and clean up the neutrophil damage. Lymphocytes (Natural Killer cells) will work on killing cells already infected with the virus. Some influenza strains have adapted to avoid being detected by these lymphocytes.
It is important to note that these immune responses frequently damage pulmonary structures.
