Intracellular antibody immunity.

Antibodies allow the immune system to target pathogens despite their tremendous diversity and rapid evolution. Once bound to a pathogen, antibodies induce a broad range of effector mechanisms, including phagocytosis and complement. However, these mechanisms are all initiated in the extracellular space, meaning that pathogens like viruses evade them upon infection of their target cells. Recently, it has been shown that, in addition to mediating extracellular immune responses, antibodies also activate immunity inside infected cells. Antibodies that are bound to the surface of non-enveloped viruses or bacteria are carried into the cell during pathogen entry. Once inside the cell, these pathogen-attached antibodies are recognised by a highly conserved, high affinity cytosolic antibody receptor called TRIM21. TRIM21 initiates both sensor and effector responses that reduce viral replication and induce an antiviral state. These responses are an important part of antiviral immunity and the removal of TRIM21 results in uncontrolled viraemia and death in a mouse model of infection.