Omega-3 fats affect human health in two basic ways. First, Omega-3 fats are important because they lead to the production a specific group of compounds called eicosanoids (pronounced eye-co-san-oids). Eicosanoids have extremely important actions affecting such things as immune response, blood pressure, blood clotting, body temperature and cell growth. Each type of eicosanoid has a different effect on a different part of the body and the type of eicosanoid produced depends on the proportion of omega-3 fats compared to omega-6s (since they compete with each other). Eicosanoids have hormone-like effects, but are made and used by the same cell (hormones are made in one cell type but work in another). In addition, eicosanoids don't function the same in different types of cells and they can influence hormone function. Here is an example of the difference between omega-6 and omega-3 effects.
Arachidonic acid, a 20 carbon, highly unsaturated fat) is acted upon by an enzyme in platelets to form an eicosanoid called thromboxane2. This compound causes blood to clot. However when that same enzyme acts upon EPA, thromboxane3 and NOT thromboxane2 is formed; thromboxane3 has less blood clotting activity. This decreased activity in combination with less thromboxane2 means much less blood clotting and may even lead to an increased tendency to bleed easily.
Another way omega-3s affect us is by being incorporated into cell membranes and other cell molecules and altering their shape or conformation. This is important because a significant amount of body function is accomplished with lock and key type of interactions. Imagine having to unlock several gates with different keys to get to a designated spot. If one of the locks is broken or bent, the key may not work and your task is cut short Omega-3s may lead to a change in the bent lock so that the key can fit, or they may alter the key or even help inactivate a lock so that an undesired destination is blocked. One example of the importance of structure on function related to omega-3s is the role of DHA in vision. Rhodopsin is a membrane protein, needed for vision, that works better if it fits nicely with the lipid surrounding it in the membrane. When the lipid is DHA, rhodopsin is more functional, hence vision is better.
As another example, one type of fat found in the brain of all mammals, called cephalin, will assume a shape that allows passage of needed substances when the fat is made up of a lot of DHA.