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Immune Response infoaging guides BIOLOGY OF AGING IMMUNE RESPONSE An introduction to aging science brought to you by the American Federation for Aging Research The immune system is composed Lymph nodes Bone marrow of an interdependent set of organs These small oval structures are The bone marrow is the home of and cells, as well as the proteins found along the lymphatic system. blood stem cells, the cells that and other chemicals they produce. Immune cells collect in them, and give rise to the various circulating Following is a brief review of what they filter out foreign substances. blood cells. The marrow produces these components are and what In inflammation and infection, red blood cells, which carry oxy- they do. they enlarge and are sometimes gen to our tissues, and platelets, referred to as “swollen glands.” small cells critical for proper blood ORGANS OF THE They enlarge because the immune clotting. The marrow also pro- IMMUNE SYSTEM cells in them divide very rapidly, duces white blood cells, of which in order to multiply and defeat there are several different kinds, Skin and mucous membranes infection. including B cells (B lymphocytes), The skin is a critical barrier that which make antibodies. White helps to prevent invasion by Thymus blood cells are the critical immune germs and foreign substances. The thymus is an organ in the cells of the body. The various mucous membranes chest, under the breastbone. It that come into contact with the produces immune cells called CELLS OF THE outside world also serve to protect T cells and is important in their IMMUNE SYSTEM against invasion. Saliva, tears, and maturation. The thymus is critical mucus contain antibacterial sub- for the development of the im- The various white blood cells stances. The small hairs or cilia mune system of infants. It also has include granulocytes, lympho- that line the trachea (windpipe) important functions in adult im- cytes, and monocytes. beat upwards to move foreign munity, but adults can live without Granulocytes matter that has been inhaled a thymus. The thymus drastically These constitute about half of your back out of the respiratory tract. shrinks with age and produces circulating white blood cells. They Germ-killing cells line the mucous only very small numbers of new contain granules, or little spots membranes and skin and prevent T cells after puberty, potentially visible under a microscope. Types germs from establishing them- setting the stage for T-cell aging. of granulocytes include: selves as infections. Also, mucous New research shows that while membranes, and in particular the adults can live without the thymus, • Neutrophils those in the gut, are densely popu- removing thymus in early child- These are the most common lated by useful microorganisms hood may accelerate the decline in white cells we have. They live (gut flora), which not only help us immunity with aging. less than a day, and their job digest food and maintain health is to react immediately to the but also serve as a barrier against Spleen presence of foreign matter. harmful microorganisms. The spleen is an organ in the They locate the foreign matter upper left quadrant of the through chemotaxis, a re- Lymphatic system abdomen that filters out foreign sponse to the release of chem- Lymph is a clear, watery fluid cells. It is an important con- icals in the presence of foreign which bathes cells of the body tainment area for white blood substances. They will surround and carries nutrients to them. After cells called lymphocytes. It is a germ or foreign body (such supplying the cells with essential particularly important in protecting as a splinter), engulf it, and re- substances, lymph collects waste against infection with certain lease other chemicals such as and foreign materials and conveys bacteria, such as Pneumococcus, hydrogen peroxide to destroy them to the lymphatics, vessels that have cell capsules— it. Pus is simply a large volume that lead to the lymph nodes. After anatomical structures, layered of dead neutrophils and foreign being filtered through the lymph over the bacteria’s cell wall. Cell matter. nodes, lymph re-enters the blood capsules make germs slippery stream as plasma. This liquid and harder to kill. While a person • Eosinophils part of the blood is composed of can live without a spleen, he or These granulocytes account lymph, rather than of red and white she would be at a higher risk of for one to three percent of all blood cells. Plasma carries nutri- developing certain infections. white cells in the blood. Their ents from the blood to the cells. numbers rise in the presence of certain parasites. However, in the absence of parasites, 2 | Infoaging Guide to Immune Response hey also participate in allergic responses. • Basophils These cells carry the chemical histamine, the source of the woes associated with allergies (hence the value of antihis- tamines). They participate in inflammation. Lymphocytes These white blood cells are called into action to defeat bacterial and viral infections. There are two ma- jor categories of lymphocytes: B lymphocytes and T lymphocytes, often called simply B cells and T cells. Unlike granulocytes and monocytes, which will respond to any infection, each T or B cell is highly specific and will respond Antibodies are shaped like the letter Y. The very tips of the only to one or a handful of similar arms of the Y vary and can stick to the foreign invader at microorganisms. which they are aimed. • B cells B cells mature in the bone Monocytes Y. There are five major forms of marrow. They “recognize” These cells are produced in the antibodies or immunoglobulin specific microorganisms, (i.e. bone marrow and released into (abbreviated Ig): IgG, IgD, IgE, bacteria , viruses, etc.), and in the circulatory system. They make IgA, and IgM. The base of the the presence of those specific their way to various organs in Y-shaped protein is similar for organisms, reproduce rapidly the body and mature into mac- all molecules within these five as identical clones. B cells rophages. Macrophages “eat” categories. The very tips of the eventually mature into plasma foreign matter such as bacteria, arms of the Y vary and can stick to cells, which are responsible parasites, inhaled fibers and dusts, the foreign invader at which they for producing antibodies, and anything else they find in are aimed, making them easier for infection-fighting proteins also organs that does not belong there, macrophages to find. specific to the particular germs including pus. Unlike neutrophils, that have invaded the body. they are hardier and much larger, • IgD able to “eat” more efficiently and This immunoglobulin is found • T cells without being destroyed them- on the surface of B cells and T cells. T cells also come in selves in the process. is involved in activating them subcategories. The two main to participate in the immune types are the killer (or cytotox- PROTEINS AND CHEMICALS OF response. ic) T cells, which can kill tumor THE IMMUNE SYSTEM cells and virus-infected cells, • IgE and helper T cells, which pro- The various immune cells This immunoglobulin partici- vide help to the killer T cells, to make a variety of proteins and pates in the allergic response. B cells, and to macrophages chemicals that are also important • IgA (see below). Some T cells also components of the immune and IgA molecules generally come serve as regulators to stop inflammatory responses. These in pairs and are found in body immune responses before they include: secretions, such as saliva. go too far. Like B cells, they Antibodies or immunoglobulin They are also found inside the “recognize” specific microor- Antibodies are proteins, made stomach and intestines. ganisms. by B cells, shaped like the letter Infoaging Guide to Immune Response | 3 • IgM IgM molecules gather in The human immunodeficiency virus, or clusters and are involved in HIV, destroys helper T cells, limiting the killing bacteria. body’s ability to fight infection. Cytokines and chemokines These are chemicals that are secreted by cells of the immune system. Chemokines are chemi- cal messengers that recruit other immune cells to where they are needed to fight infection and even cancers. Cytokines are used to augment or diminish the im- mune response and inflammation, and some can also directly kill or inhibit germs. Because they communicate between white blood cells, also called leukocytes, cytokines are sometimes called interleukins. Complement proteins These proteins are manufactured in the liver. They circulate in the bloodstream and are activated by the presence of antibodies (thus they complement their activity). They act to puncture and thus burst, or lyse, foreign cells. Antigens DNAAntigens damage are the alsochemicals contrib on -the Autoimmune disease AIDS The loss of the recognition of The human immunodeficiency utessurfaces to theof microorganisms development or of foreign cells that are recognized some body tissue as “self” is part virus or HIV destroys helper T age-relatedby the immune diseases, system. Antibodies such of autoimmune disease. Antibod- cells, limiting the body’s ability to asare heartcreated disease, to specific lung antigens. can- ies and T cells attack the body’s fight infection. B cells have antigen-receptor own tissue. Some examples of this Cancer and its treatments cer,sites andon their atherosclerosis. surfaces, which are include some forms of diabetes, Cancer is a disease of out-of-con- proteins that recognize those an- rheumatoid arthritis, and lupus. trol cell proliferation. Sometimes tigens. T cells have more sophis- Immune complex disease this disease becomes manifest ticated antigen-sensing systems Normally, the clumps of antibodies because of a failure of the immune and do not recognize “naked” and antigens that form as we fight system to destroy cancerous cells.
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