Arachidonic acid is a fatty acid that is a common component of phospholipids. Phospholipids can be used as a source of arachidonic acid when it is metabolised within cells in an enzyme-catalysed pathway known as the cyclooxygenase (COX) pathway. The final products of the COX pathway can be different in different cell types, causing a range of responses. In some cells, the products are involved in the inflammatory response, which is a response by the body to infection. In other cells, cell division is stimulated. Fig. 5.1 shows the first reaction in the COX pathway. This reaction is catalysed by an enzyme known as COX-2. [Figure 5.1]