Inhibition A reduction in the activity of a reaction.  In toxicokinetics, it normally refers to enzyme inhibition. Interstitial fluid The fluid in the space between cells.  Same as intercellular fluid. Intracellular Fluid The fluid within a cell.  It is also known as the cytoplasm. Ionized Separated into ions.  Normally, an ionized substance will dissolve in water. Irritation Local tissue reaction without involvement of an immunologic mechanism.  It is a reversible inflammation. Interactions Refers to measures of effects of simultaneous exposure to two or more substances.  The four types of interactions are additive, antagonistic, potentiation, or synergistic.