According to the dictionary at Biology-Online.org, cellular irritability describes the ability of any cell to recognize and react to stimuli in its immediate surroundings. In a pathological sense, this recognition ability is under-regulated, which results in excessive amounts of the body's sensitivity to certain stimuli.
Biology-Online.org clarifies that only some cells in the body actually exhibit irritability under normal conditions. These cells include muscle cells, neurons and endocrine cells. Each of these cells relies heavily on intercellular communication in order to achieve their specialized functions, according to Wikipedia. Similar failures in cellular signaling regulation can also result in many cancers.