NK-natural killer identifies the abnormalities on cells by detecting the amount of what? a. MHC I molecules b. Non-self -molecules c. PAMP-pathogen -associated molecular patterns d. PRR-pattern recognition receptors

The correct answer is a. MHC I molecules.<br /><br />Natural killer (NK) cells are a type of cytotoxic lymphocyte that play a crucial role in the innate immune system. They are responsible for identifying and eliminating abnormal cells, such as virus-infected cells or tumor cells, in the body.<br /><br />NK cells recognize and target cells that have downregulated or lost their major histocompatibility complex (MHC) class I molecules on their surface. MHC class I molecules are responsible for presenting peptide fragments derived from intracellular proteins to the immune system. When a cell becomes infected with a virus or undergoes transformation into a tumor cell, it often reduces the expression of MHC class I molecules on its surface. NK cells detect this loss of MHC class I molecules and recognize the cell as abnormal or "selfish," leading to its destruction.<br /><br />Therefore, NK cells identify abnormalities on cells by detecting the amount of MHC class I molecules present on their surface.