Innate and Adaptive Immunity: Cells and Functions
Natural Killer (NK) Cells
NK cells attack and destroy tumor cells and virally infected cells through a process known as “self-extinction.” This term describes cells with low levels of a cell surface marker called MHC I (major histocompatibility complex) – a situation that may arise in viral infections of host cells. Such cells are called “natural killer” because of the initial concept that they did not require activation to kill cells that were “missing self.” Like other “unconventional” T cells which bear unchanging T cell receptors (TCRs), such as CD1d-restricted natural killer cells, these cells exhibit characteristics that place them on the border between adaptive immunity and innate immunity. On one hand, T cells could be considered a component of adaptive immunity because they modify genes for TCR recombination, producing a cross-memory phenotype. However, the many combinations could also be considered part of the immune system where the restricted TCR or NK cell receptors could be used as a pattern recognition receptor. For example, according to this paradigm, large amounts of Vγ9/Vδ2 T cells respond within hours to common molecules produced by microbes, and highly elevated Vδ1 T cells in the intraepithelium respond to stressed epithelial cells.
Neutrophils
Neutrophils, like two other cell types – eosinophils and basophils (see below) – are known as granulocytes due to the presence of granules in their cytoplasm, or polymorphonuclear cells (PMNs) due to their distinctive lobed nuclei. Neutrophil granules contain a variety of toxic substances that kill or inhibit the growth of bacteria and fungi. Similar to macrophages, neutrophils attack pathogens by activating a respiratory burst. The main products of the neutrophil respiratory burst are strong oxidizing agents, including hydrogen peroxide, free oxygen radicals, and hypochlorite. Neutrophils are the most abundant type of phagocytic cell, typically representing 50 to 60% of the total circulating leukocytes, and are usually the first cells to arrive at the site of an infection. The bone marrow of a normal, healthy adult produces over 100 billion neutrophils per day, and more than 10 times that many during acute inflammation.
Dendritic Cells
Dendritic cells (DC) are phagocytic cells present in tissues that come into contact with the external environment, mainly the skin (where these cells are named Langerhans cells) and the mucosal lining of the nose, lungs, stomach, and intestine. They are named by analogy with neuronal dendrites, but dendritic cells are not connected to the nervous system. Dendritic cells are important in antigen presentation and serve as a liaison between the innate and adaptive immune systems.
Basophils and Eosinophils
Main article: Eosinophilic granulocyte
Basophils and eosinophils are cells related to neutrophils (see above). When activated upon encountering a pathogen, basophils release histamine, which is important in the defense against parasites and plays a role in allergic reactions (such as asthma). Upon activation, eosinophils secrete a range of highly toxic proteins and free radicals that are highly effective in killing bacteria and parasites, but they are also responsible for the tissue damage that occurs during allergic reactions. The activation and release of toxins by eosinophils is, therefore, strictly regulated to prevent any damage from improper tissue destruction.
Natural Killer Cells
Natural killer cells, or NK cells, are a component of the innate immune response. NK cells attack host cells that have been infected by microbes, but they do not directly attack invading microbes. For example, they target and destroy tumor cells and virally infected cells.