Host cells recognize a pathogen through viral sensing. The epithelial cells are the first line of defense by secreting lysozyme and lactoferrin. This, in turn, induces interferons that act to reduce or limit the virus's spread to other cells.
Some of these interferons activate immune cells that will start to phagocytize viruses, while other interferons released from dendritic cells will stimulate T cells to secrete cytokines and chemokines. These interferons can impair the ability of the virus to enter the cell, which prevents the virus from replicating. Still, other interferons will 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 are able to 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 that have already become 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.
