Natural killer lymphocytes, or NK cells, rapidly respond to virally infected cells and tumor formation, typically within three days. They cause lysis or apoptosis, often in response to virus-infected cells removing MHC1 receptors from their cell surface.
Neutrophils circulate in blood and are phagocytic cells. They ingest pathogens and are called to infected sites through chemical signaling. They move via chemotaxis, following the concentration gradient of cytokines. They are the predominant cells in pus and account for its white/yellow appearance.
Host cells make the protein interferon in response to infection by viruses, bacteria, parasites, or the emergence of a tumor cell. Interferons are cytokines that start the signaling cascade of the immune response.
The complement system is part of the innate immune system and complements the abilities of antibodies or phagocytic cells to remove pathogens. Complement proteins lyse open infected cells or pathogens and kill them. The debris is usually processed by phagocytic molecules.
Dendritic cells are antigen presenting cells. They present an antigen on the cell surface so that the adaptive immune system can generate the appropriate specific response to the pathogen that the antigen came from.
Macrophages exist in the interstitium of tissue. They are phagocytes, engulfing and digesting intercellular debris and pathogens. They also signal other parts of the immune system through cytokines.
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