When You're the One Canceling a Flight. In defense of Spirit, most airlines operate on razor thin margins and refund dollars do add up. Even airlines that allow refunds-generally only under very special circumstances-may make you jump through hoops and even pay a fee.
Medical Definition of receptor. 1  1: a cell or group of cells that receives stimuli : sense organ. 2  2: a chemical group or molecule (as a protein) on the cell surface or in the cell interior that has an affinity for a specific chemical group, molecule, or virus.
Introduction. Natural killer (NK) cells are a sophisticated cell of the innate immune system with the capacity to kill cells and produce cytokines through the interaction of a variety of cell surface receptors with their cognate ligands.K cell surface receptors regulating effector function. Natural killer cells express a number of receptors that activate or inhibit effector function. These receptors belong to either the Ig-superfamily of type I membrane proteins (Table 1) or the C-type lectin superfamily of type II membrane proteins (Table 2).