Final answer:
Natural killer (NK) cells are a part of the innate immune system known for their role in rapidly attacking pathogen-infected cells and tumors without the need for antigen sensitization, working alongside cells of the adaptive immune system, especially against viral infections.
Step-by-step explanation:
Natural killer (NK) cells are indeed part of the innate immune system. This is true because, unlike cells of the adaptive immune system such as T cells and B cells, NK cells do not require prior sensitization to a specific antigen to become active. They are capable of recognizing and killing certain pathogen-infected cells and tumor cells without the need for prior activation through specific antigens. This is in contrast to the adaptive immune system, where cells need to recognize specific antigens presented by other immune cells in order to respond.
Natural killer cells work in conjunction with the adaptive immune system, such as cytotoxic T cells, particularly against virally infected cells. They also participate in the body’s first line of defense against viruses and certain types of cancer. In terms of immune response, NK cells are important as they can respond more rapidly than the adaptive immune response, which requires time to develop after an initial exposure to a pathogen.
Additionally, there are indeed multiple cell types within the immune system capable of phagocytosis, with macrophages being a classic example. It is important to note that not all leukocytes in the innate immune system are phagocytic: NK cells kill foreign cells directly, rather than through phagocytosis.