Final answer:
The immune system uses antibodies, cytotoxic T cells, and cells with pattern recognition receptors like Toll-like receptors to detect and respond to viruses, even after they have infected host cells.
Step-by-step explanation:
The immune system detects a virus that has infected a cell using a variety of mechanisms. Viruses have specific binding sites on their surfaces which facilitate entry into host cells that possess compatible receptors. Once inside, certain viral components, such as antigens, are recognized by the immune system, prompting the production of antibodies. These antibodies attach to viruses to either destroy them or inhibit their activity. However, in some viruses like HIV, mutations in these binding sites lead to rapid changes making it difficult for antibodies to recognize and neutralize the evolved virus variants.
Moreover, the immune system also has T cells, including cytotoxic T cells, which can detect and kill cells that have been infected by a virus, effectively stopping the virus from replicating. These T cells are aided in their detection of infected cells by certain signaling molecules, like interferons, which slow down viral replication.
Furthermore, the immune system utilizes macrophages and dendritic cells to recognize pathogen-associated molecular patterns (PAMPs) on pathogens. They contain pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), enabling them to identify and respond to the molecular signatures of various pathogens, including viruses infecting host cells.