Immunology Terms Starting With N

Natural Killer Cell is a cytotoxic lymphocyte of the innate immune system that eliminates virus-infected and cancerous cells without prior sensitization by recognizing missing or altered self-signals on target cell surfaces.

NK cells integrate signals from an array of activating and inhibitory receptors to decide whether to kill a target cell. Inhibitory receptors including KIR and NKG2A recognize MHC class I molecules that healthy cells normally display at their surface. Tumor cells and virus-infected cells that downregulate MHC class I to evade cytotoxic T cells thereby become vulnerable to NK cell killing, a strategy described as “missing self” recognition first articulated by Klas Kärre in 1981.

Activated NK cells kill targets by releasing perforin and granzymes and by engaging death receptors on the target cell surface, typically completing a killing event within 30 minutes of contact.

Neoantigen is a tumor-specific peptide antigen arising from somatic mutations in cancer cells that is recognized as foreign by the immune system and absent from normal, unmutated tissues.

Neoantigens result from somatic mutations in cancer cells that alter protein sequences, creating epitopes not subject to central immune tolerance. A typical melanoma harbors 100 to 400 somatic mutations, but only 1 to 10 percent generate neoantigens capable of eliciting T cell responses, because most mutated peptides bind MHC molecules poorly or fail to engage T cell receptors with sufficient affinity. These mutation-derived peptides are processed and presented on MHC class I molecules, allowing CD8-positive cytotoxic T cells to distinguish cancer cells from healthy tissue with high specificity.

Neoantigen burden predicts response to immune checkpoint inhibitors: patients whose tumors carry higher mutational loads show response rates exceeding 60 percent compared to roughly 20 percent in low-mutation tumors. Personalized cancer vaccines targeting patient-specific neoantigens have entered clinical trials, with early studies reporting induction of neoantigen-specific T cells in over 95 percent of vaccinated melanoma patients.

Neutrophil is the most abundant circulating white blood cell and the first responder to bacterial and fungal infections, destroying pathogens through phagocytosis, oxidative burst, and release of antimicrobial granule contents.

Neutrophils are produced at a rate of roughly 100 billion per day in adult human bone marrow and survive only six to twelve hours in circulation before undergoing apoptosis, making them short-lived but numerically dominant effectors. Recruited rapidly to infection sites by chemokines and complement fragments, they engulf and destroy microbes through reactive oxygen species, antimicrobial peptides such as defensins, and neutrophil extracellular traps, which are web-like structures of decondensed chromatin coated with antimicrobial proteins. Chronic neutropenia caused by chemotherapy or primary immunodeficiency produces profound susceptibility to bacterial and fungal infections, confirming their indispensable contribution to early host defense.

A single neutrophil can phagocytose and kill roughly 5 to 20 bacteria before its own antimicrobial capacity is exhausted.

NOD Receptor is a member of a family of cytoplasmic pattern recognition proteins that detect bacterial peptidoglycan fragments and cellular stress signals inside the cell to trigger inflammatory and antimicrobial responses.

NOD receptors comprise a family of over 20 cytoplasmic sensors in humans, with NOD1 and NOD2 being the best-characterized members. NOD2 specifically detects muramyl dipeptide, a component found in nearly all bacterial cell walls, and mutations in the NOD2 gene increase Crohn’s disease risk by 20- to 40-fold in affected individuals. Upon ligand binding, NOD receptors oligomerize and recruit the adaptor protein RIP2, triggering NF-?B activation and production of inflammatory cytokines within 30 to 60 minutes.

Unlike Toll-like receptors positioned at cell surfaces and endosomes, NOD receptors surveil the cytoplasmic compartment, detecting bacteria that have invaded cells or bacterial products translocated across membranes. Some NOD-like receptors form inflammasomes, multi-protein complexes that activate caspase-1 and process pro-IL-1? and pro-IL-18 into their active secreted forms.