The human immune system is a highly complex, bodywide network of specialized cells, organs, and even a separate circulatory system, which work in concert to clear infection from the body. The organs of the immune system are called lymphoid organs. Lymphatic vessels and lymph nodes comprise the special circulatory system that transports white blood cells to sites of infection throughout the body.
Lymph (after the Greek meaning "clear stream") is a clear-to-white fluid made of chyle (fluid from the intestines after digestion which contains proteins and fats), some red blood cells, and many white blood cells, especially lymphocytes. Lymphocytes are cells that attack bacteria in the blood. Operating in close partnership with blood circulation, lymph bathes the tissues of the body, and lymphatic vessels collect and move it back into the blood.
In addition to lymph nodes (scattered in the neck, armpits, and groin areas), the tonsils, adenoids, thymus, and spleen all perform specialized immune functions. Also, bone marrow of certain long, flat bones (such as the bones of the pelvis) produces cells that grow into the more specialized cells that circulate throughout the immune system. The appendix is also a lymphoid organ and helps protect the cecal part of the colon.
While immune-system processes are far too complicated for a lengthy discussion here, it is necessary to know that there are two major classes of lymphocytes: B cells, which mature within bone marrow, and T cells, which mature in the thymus (located behind the breastbone). B cells are part of the body's antibody-mediated, or humoral (after the Greek "humor," for the body's blood and lymph systems), immune system. B cells produce antibodies that circulate in blood and lymph and attach to antigens (foreign substances), marking them for destruction by other cells.
T cells also patrol for foreign invaders, but in addition to marking them, certain T cells also can attack and destroy cells. T lymphocytes are responsible for cell-mediated immunity (also called cellular immunity). They also coordinate overall immune-system response.
While the immune system ages right along with the rest of our body, it may actually do so at an increased rate because of the speed at which immune cells proliferate. During viral invasions or bacterial infections, the immune system mobilizes billions of white blood cells. Cells that divide rapidly run a higher risk of frequent genetic mutations, especially when free-radial generation is high. Antioxidant defenses naturally fall and DNA damage by free radicals may increase by as much as fifteen times as we age, making us even more susceptible to damage that accompanies infections.
This means that we ran a higher risk of cancer of blood-forming tissues, such as leukemia, myeloma, and lymphoma, as well as immune incompetence. Such a situation leaves the body unable to protect itself from even common infections and increases risk for autoimmune diseases. As with so many other health-related disorders, the incidence of immune-related diseases in the population appears to be increasing—and many can be correlated directly to poor nutritional practices in all age groups.
Poor nutrition does not affect all parts of the immune system equally. Most sensitive is the cellular arm of the immune system, consisting of macrophages (specialized scavenger cells), lymphocytes (T-cells), natural killer cells, and other specialized immune cells.510 Antibodies are less affected. This is exactly what we see with aging—cellular immunity that is depressed out of proportion to humoral antibodies.511
Because immune overreaction is common with aging, autoimmunity due to other factors, such as viral infections and environmental toxins, can be worse in the aged. Interestingly, autoimmunity to brain proteins is much higher in cases of Alzheimer's, Parkinson's, and Lou Gehrig's diseases (ALS).512
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