and eosinophils are cells related to the neutrophil. When activated by a pathogen encounter, histamine-releasing basophils are important in the defense against parasites and play 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 parasites, but may also damage tissue during an allergic reaction. Activation and release of toxins by eosinophils are, therefore, tightly regulated to prevent any inappropriate tissue destruction. Natural killer cells Natural killer cells (NK cells) do not directly attack invading microbes. Rather, NK cells destroy compromised host cells, such as tumor cells or virus-infected cells, recognizing such cells by a condition known as "missing self." This term describes cells with abnormally low levels of a cell-surface marker called MHC I (major histocompatibility complex) - a situation that can arise in viral infections of host cells. They were named "natural killer" because of the initial notion that they do not require activation in order to kill cells that are "missing self." The MHC makeup on the surface of damaged cells is altered and the NK cells become activated by recognizing this. Normal body cells are not recognized and attacked by NK cells because they express intact self MHC antigens. Those MHC antigens are recognized by killer cell immunoglobulin receptors (KIR) that slow the reaction of NK cells. The NK-92 cell line does not express KIR and is developed for tumor therapy. γδ T cells Like other 'unconventional' T cell subsets bearing invariant T cell receptors (TCRs), such as CD1d-restricted Natural Killer T cells, γδ T cells exhibit characteristics that place them at the border between innate and adaptive immunity. γδ T cells may be considered a component of adaptive immunity in that they rearrange TCR genes to produce junctional diversity and develop a memory phenotype. The various subsets may be considered part of the innate immune system where a restricted TCR or NK receptors may be used as a pattern recognition receptor. For example, according to this paradigm, large numbers of Vγ9/Vδ2 T cells respond within hours to common molecules produced by microbes, and highly restricted intraepithelial Vδ1 T cells will respond to stressed epithelial cells. Other vertebrate mechanisms The coagulation system overlaps with the immune system. Some products of the coagulation system can contribute to non-specific defenses via their ability to increase vascular permeability and act as chemotactic