Immune System Health Article

Central lymphoid tissues

The central lymphoid organs include the bone marrow and thymus. At these sites, the lymphocytes interact with other cells to enhance their development or increase their ability to assist in an immune response. They also acquire the ability to recognize specific antigens before they actually become exposed to them, and are antigen independent. At this stage the lymphocytes are called naïve lymphocytes because they have not yet been exposed to antigens. The bone marrow is the site of hematopoiesis. Both B-lymphocytes and T-lymphocytes come from this site, but only the B cells undergo maturation in this area (hence the name B-cell T-cell).

Peripheral lymphoid tissues

The peripheral lymphoid tissues include the lymphatic vessels, lymph nodes, various lymphoid tissues, and spleen. The events that occur in these areas require exposure to an antigen, and are called antigen-dependent events.

Lymphatic vessels

The filtration of the blood results in the production of extracellular fluid called lymph. The lymphatic vessels that carry the fluid back to the bloodstream also carries cells with antigens. These antigens come from other sites within the body where infection may be present. The fluid passes through the lymph nodes. This fluid is eventually returned to the blood via lymphatic vessels. All the lymph from the body is carried back to the heart by way of the thoracic duct.

Lymph nodes and lymphoid tissue

Lymph nodes are distributed along lymphatic vessel pathways and act as a filter for the lymph. The lymph nodes are distributed throughout the lymphatic system, and are especially prominent in the neck, axilla (underarm), and groin. These fibrous nodes contain immune cells such as lymphocytes, macrophages, and dendritic cells. Dendritic cells have long, filamentous cytoplasmic processes. These processes have the ability to bind antibodies such that the antibodies can also bind with their specific antigens. This creates a web that traps antigens. The macrophages in the lymph nodes degrade debris and

extract material that contains antigens, such as those from pathogenic bacteria. The structure of the lymph nodes is such that both T-and B-cells are exposed to this antigenic material. The cells that recognize this material are held in the lymphoid nodes and tissues where they multiply and differentiate. These cells become effector cells that are capable of fighting disease. The node may enlarge during this process, giving rise to the clinical observation of swollen glands.

Lymphocytes can also be found in several other areas throughout the body. The gut-associated lymphoid tissue is a broad term that describes lymphoid tissue found in the Peyer's patches of the intestine, appendix, adenoids, and tonsils. Cells that protect the respiratory tract are called bronchial-associated lymphoid tissue (BALT). Other mucosal areas are protected as well, and are collectively known as mucosal-associated lymphoid tissue (MALT).

Spleen

Blood is filtered in the spleen, where damaged or dead red blood cells are removed from the blood as well as antigens. This organ also serves as a site for storage of erythrocytes and platelets. In the fetus, it is the site of erythropoiesis (formation of red blood cells). Within this organ reside B-cells, T-cells, macrophages, and dendritic cells. As in the lymph nodes, lymphocytes are trapped in this organ. Antibodies and effector cells are produced in the spleen.

Common disorders and diseases

Hypersensitivity reactions result from an immunemediated inflammatory response to an antigen that would normally be innocuous (causing no harm to the body). Examples include allergic reactions, such as hay fever, asthma, reactions to insect bites, and the systemic anaphylactic shock that occurs in response to bee stings, allergies to antibiotics, and foods.

Delayed-type hypersensitivity reactions are due to the release of lymphokines. These lymphokines are small polypetides produced by lymphocytes that have been stimulated by an antigen, affecting other cells. This hypersensitivity reaction may occur as part of the normal immune response to infection by bacteria and viruses. This effect is responsible for the tissue damage in the lungs due to tuberculosis, the skin lesions that occur in leprosy and herpes, and rashes associated with chicken pox and measles. This may also occur via skin exposure to cosmetics, poison ivy, and allergy to metals in jewelry, resulting in contact dermatitis.

Autoimmune diseases occur when the immune system begins to attack the body or "self." In Grave's disease, antibodies are produced against the thyroid-stimulating hormone (TSH) receptor. In multiple sclerosis (MS), antibodies are produced against elements of the myelin sheaths in the brain and spinal cord. The effects of myasthenia gravis are traced to antibodies directed against the acetylcholine receptor. Following a heart attack, antibodies may form against heart muscle antigens resulting in autoimmune myocarditis. Rheumatoid arthritis (RA) develops from complexes pf antibodies to immunoglobulin G (IgG) in the joints and connective tissue. In systemic lupus erythematosus, the body produces antibodies directed against nuclear antigens and DNA.

In acquired immunodeficiency syndrome (AIDS), the HIV retrovirus attacks T-cells (CD4), dendritic cells, and macrophages. The number of CD4 T-cell in the blood eventually declines and the body can no longer resist the HIV infection. With the immune system compromised, constitutional disease can develop with fever, weight loss, or diarrhea. Neurological disease can occur, resulting in dementia and effects to the peripheral nervous system. Pathogenic microorganisms may cause opportunistic infections in this compromised immune state, such as pneumonia, diarrhea, skin and mucous membrane infections, and central nervous system infections. Cancers may also arise, such as lymphomas. Death from HIV is due to one of these complications or a combination of effects.