Several types of dietary lipids (fatty acids) have been shown to modulate immune function. Polyunsaturated fatty acids (PUFAs)—fatty acids with more than one double bond between carbons—are broadly divided into two main classes: omega-3 PUFAs and omega-6 PUFAs. Lipids of both classes are essential nutrients because they cannot be synthesized by the body; therefore, PUFAs must be obtained from the diet. Dietary sources of omega-3 PUFAs include oily fish, flaxseeds and their oil, walnuts and their oil, and canola oil. Food sources of omega-6 PUFAs include vegetable oils (e.g., soybean, safflower, and corn oil), nuts, and seeds.