Humans can look in the mirror and inherently recognize the self—an ability not given to all living beings. And as humans have evolved into complex organisms, so have the microorganisms floating around in the air, or resting invisibly on the nearest door handle. Thankfully, the human immune system has also evolved and is able to recognize the self on a cellular level. Small battles take place regularly with invaders—or antigens—in the form of bacteria, viruses or parasites. The human body is an ideal place for antigens to reside, and if these invaders manage to get past the body’s first lines of defense—the skin and mucosal lining of the digestive and respiratory tracts—an elaborate chain reaction initiates, and the body deploys specialized cellular troops to contain and oust the enemy. Immunity is a team effort among several systems, which each produce efficient cells for specific immune function duties.
Also, the body doesn’t have to fight alone. Advances in technology have shed light on how various vitamins, minerals, whole foods and other compounds can contribute to immune health. Life expectancy, at least in civilized countries, has drastically increased in the last hundred years because of great strides in food science technology, knowledge dissemination, and the resultant opportunity for humans to shore up natural immunity. Humans can now concentrate on preventing immune system breakdown via a smart diet and habitual supplement regimen. And this, just as with immune function, can be a multi-pronged effort.
A Well-Oiled Machine
Let’s assume an infectious microbe has penetrated the body’s outer defenses. Normal cells are equipped with two classes of distinctive surface proteins, MHC Class I proteins, which are on all cells, and MHC Class II proteins, which are only on certain specialized cells. Any cells not bearing such proteins, called antigens, initiate an immune response. The lymphoid system assists the assault on invading antigens, offering lymphocytes—white blood cells specifically produced to help the body avoid disease. Well known organs such as the spleen, thymus, appendix and tonsils join the fray, as do less obvious players such as lymph nodes and bone marrow.
Cells biologically marked to become immune cells originate as stem cells in bone marrow. Some develop into myeloid progenitor cells, while others become lymphoid progenitor cells. From there, these cells are specialized even further—myeloid cells can become monocytes, erythrocytes, eosinophils, mast cells, basophils, neutrophils or platelets. Lymphoid cells, on the other hand, become B cells, T cells, dendritic cells (macrophages) or natural killer (NK) cells.
B cells secrete soluble substances known as antibodies. Working hand in hand with T cells, B cells consume antigens and begin churning out millions of identical antibodies to fight the invader. Antibodies belong to a family of large protein molecules known as immunoglobulins. There are currently nine chemically distinct classes of human immunoglobulins—four kinds of IgG and two kinds of IgA, plus IgM, IgE and IgD. Each plays a role in the immune response, making up one part of the body’s natural fortress. T cells contribute to immunity in two ways: some play a regulatory role in the workings of the overall immune response, while others are cytotoxic and directly contact infected cells and destroy them—why they’re sometimes called killer T cells.
Helper T cells are a common form of T cells and are needed to activate many immune cells, including B cells and other T cells. From mature T cells come cytokines—interleukins, growth factors and interferons—responsible for communicating with other immune cells. Lymphocytes secrete cytokines called lymphokines, while the cytokines from monocytes and macrophages are called monokines. Many cytokines are also known as interleukins because they serve as messengers between leukocytes, immune warriors from white blood cells. Cytokines enlist the help of other cells for a more effectual immune response; they also encourage cell growth, promote cell activation, direct cellular traffic, and destroy target cells.
Two more specialized varieties of lymphocytes are killer cells— cytotoxic T cells and natural killer (NK) cells. Both cell types are produced to destroy invaders by recognizing them as foreign and delivering destructive chemicals. Cytotoxic T cells need to recognize a specific antigen bound to self-MHC markers before doing their job, whereas NK cells will recognize and attack cells lacking these proteins. Other cells geared for destruction of antigens are phagocytes, including monocytes and granulocytes, eosinophils, basophils and macrophages.
Also important in the immunity equation is the complement system, which consists of a series of about 25 proteins that “complement” the work of antibodies in destroying bacteria. This system also helps rid the body of antigen-antibody complexes. In fact, complement proteins are what cause blood vessels to become dilated and leaky, resulting in redness and swelling during an inflammatory response. The complement system embodies the actions of the entire immune response and earns its name from each specialized piece working with the others. The end product is a cylinder that punctures the target cell’s membrane, allowing fluids and molecules to flow in and out, which ultimately destroys it.
Swelling and redness are external signs of inflammation from a traumatic event. But inflammation happens internally, too, bringing with it uncomfortable symptoms such as pain and stiffness. Inflammation is the sensory manifestation of the immune response. Inflammation occurs when a group of cells (tissue) is injured by bacteria, trauma, toxins, heat or any other cause. That trauma is then handled by various specialized cells, or mediators, the third series of which involve substances called prostaglandins.
Prostaglandins’ main role in the inflammatory response is attracting neutrophils, the same role complement proteins play. Neutrophils answering the call for inflammation control come first from the bloodstream and second from bone marrow. Neutrophils contain small lysosomal granules (enzymes) which digest substances the neutrophils surround. Larger macrophages called monocytes follow soon after to help neutrophils marginalize the cause of inflammation, a common immune process called phagocytosis.