Blood transports oxygen and nutrients to all parts of the body and carries away carbon dioxide and other waste materials. Oxygen is carried inside red blood cells, which make up almost half the volume of blood. Other components of blood include white blood cells, which combat infections, and platelets, tiny cells that help to form blood clots to seal damaged blood vessels.
Disorders of the blood may be caused by an abnormality in the number, content, or form of one or more of the different types of blood cell. The first articles in this section deal with the different types of anaemia and with sickle-cell disease. In these disorders, haemoglobin, the oxygen-carrying pigment in red blood cells, is deficient or abnormal, or red cells are destroyed at an accelerated rate. Disorders in which the blood either fails to clot or clots too readily are then discussed. Some of these disorders are inherited, such as haemophilia, Christmas disease, and von Willebrand’s disease. In these disorders, the genes that are responsible for producing specific clotting factors are either absent or abnormal. Various cancers of the blood, called leukaemias, are also discussed. These disorders result from an overproduction of white blood cells, which suppresses the production of the normal blood cells in the body. The final article in this section deals with polycythaemia, a condition in which too many red blood cells are produced.
For more information on the structure and function of blood, see Formation of Blood Cells.
The lymphatic system consists of lymph nodes, or glands, connected by a network of lymphatic vessels that extends to all parts of the body. The system drains a fluid called lymph from the body’s tissues back into the bloodstream. It also protects the body against infection and the development of cancer by filtering out infectious organisms and cancerous cells from the lymph.
Lymph, a fluid that contains white blood cells called lymphocytes, fats, and protein, flows along the network of lymphatic vessels and ultimately re-enters the bloodstream from which it originally came. Along the way, lymph is filtered through the lymph nodes, which are located in clusters close to the skin’s surface in the neck, armpits, and groin. Nodes are also found deep within the abdomen and chest. The lymph nodes contain large numbers of lymphocytes, white blood cells that can destroy infectious organisms or cancer cells. Lymphocytes can also develop into cells that produce antibodies, which help other white blood cells trap and destroy infectious organisms and cancer cells throughout the body.
The first article in this section covers lymphadenopathy, in which the lymph nodes become enlarged. A painful swelling of the lymph nodes is not often a cause for concern, but painless swelling may be a sign of a serious underlying disorder, such as cancer. The next articles cover lymphangitis, in which lymphatic vessels are inflamed, and lymphoedema, in which lymph cannot drain from a limb, causing the limb to swell. In developed countries, a common cause of lymphoedema is surgery or radiotherapy for cancer. The final article discusses lymphomas, a group of cancers that may develop in one or more lymph nodes.
For more information on the structure and function of the lymphatic system, see Formation of Blood Cells.
Our immune systems protect us from the threat of infection by microorganisms, such as bacteria and viruses, and parasites, such as worms. The immune system also protects us from certain cancers and helps to repair damaged tissues. We are born with certain built-in defence mechanisms, but much of our immunity is acquired as we are exposed to the organisms that cause disease.
This section begins with an overview of acquired immunodeficiency, one of the major disorders of the immune system. Acquired immunodeficiency is a condition, developing after birth, in which the immune system fails to function properly and is unable to combat microorganisms that invade the body. Such immunodeficiency develops for a variety of reasons. The most widely publicized form of acquired immunodeficiency is AIDS, which develops after infection with the human immunodeficiency virus (see HIV infection and AIDS).
A temporary, mild form of acquired immunodeficiency may appear after infection with other viruses, such as measles, and may also develop following certain drug treatments.
The second overview article covers autoimmune disorders, which occur when a person’s immune system functions abnormally and starts to attack the body’s own tissues, causing certain organs to become inflamed and damaged. Individual autoimmune disorders that affect more than one organ are covered next. Such disorders are most common in young adults and can lead to persistent poor health. Their causes are not fully understood, although genetic factors appear to play an important role. Overall, women are affected much more commonly and severely by autoimmune disorders.
Autoimmune disorders that affect specific organs or sites in the body are considered in the relevant sections of this guide. For example, rheumatoid arthritis, which primarily affects the joints, is dealt with in the musculoskeletal disorders section and Hashimoto’s thyroiditis (see Thyroiditis), which causes the underproduction of hormones from the thyroid gland, is covered in the section dealing with metabolic disorders. Immunodeficiency that is present from birth is covered in the section of the guide dealing with children’s disorders (see Congenital immunodeficiency).
An allergy is an abnormal reaction of the body’s immune system to a foreign substance. In most people, the substance produces no symptoms, but in a susceptible person it triggers an allergic reaction. Most allergies are mild and merely unpleasant, and they are easily treated with drugs and self-help remedies. However, sometimes allergies can be life-threatening.
In this section, allergic reactions due to foreign substances such as pollen, food, and certain drugs are discussed first. The trigger substance (allergen) causes no symptoms on initial contact. However, the immune system begins to form antibodies against the allergen, and certain types of white blood cells become sensitive to it. Later contact with the allergen may stimulate specialized cells called mast cells to release histamine, the chemical that triggers the allergic response. Allergic reactions may lead to urticaria or angioedema, which are dealt with next. The last article covers anaphylaxis, a potentially life-threatening reaction to an allergen. Allergies often develop in childhood and may either persist or disappear in adulthood. Conditions such as asthma and eczema, which may have an allergic basis, are dealt with in the sections that cover the relevant body systems.
For more information on the allergic response, see Preventing Indigestion.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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