Beta Glucans

Beta glucans are substances derived from carbohydrates, similar in structure to starch and cellulose but very different in their effect on the body. The main beta glucan used in natural therapy is beta 1-3/1-6 glucan (referring to details of their chemical structure) derived from yeast cell walls; this obviously uses a cheap starting material (brewers yeast) but is a lot more expensive than one might expect from that, because the extraction process is complex.
Beta glucan's main effect on the body is improvement of the immune system. Unlike the case of many other immune system stimulants, this does not lead to an over-stimulated immune system, because beta glucan works by making phagocytes (which are immune system cells whose function is to trap and destroy foreign substances in our bodies such as bacteria, viruses, fungi, and parasites) and natural killer cells, two major components of the immune system, work more efficiently.
Beta glucans also reportedly lower elevated levels of LDL cholesterol, aid in wound healing, and help in the prevention and treatment of cancer.
Although this is not particularly relevant to many people, it also ought to be noted that beta glucans also help limit the bad effects of exposure to X-rays and nuclear radiation; this is probably because the immune system becomes more able to spot the cells damaged by these and destroy them. (Cellular destruction to some degree is inevitable, necessary and useful.) Beta glucans work, in this use, by stimulating the growth of blood-forming stem cells in bone marrow - drastic slowdowns in growth of various blood cells are the most obvious damage caused by radiation.
Beta glucans are immune system modulators rather than stimulators. The difference is that beta glucans can help in various allergies, where the problem is that the immune system is responding inappropriately, and also in arthritis, which has similar roots.