Diseases and Disorder of the Skin

Any component of the skin can become involved in a variety of different diseases. The skin can often provide doctors with clues about the health of the body, since many diseases that affect other organ systems are evident as telltale clues on the skin. For example, a rash, such as that typical of measles or chicken pox, can indicate an infection that affects the whole body. A blue tinge to the skin, called cyanosis, means that the body is not getting enough oxygen and can indicate heart failure. A yellow tinge to the skin, known as jaundice, signals that the liver is not working properly. Other diseases are specific to the skin itself. These disorders range from the merely annoying or distressing, such as acne, to the potentially life-threatening, such as skin cancer.

In response to various triggering substances, or allergens, immune cells known as mast cells in the skin may release a chemical called histamine. Other immune cells called T cells may directly damage the epidermis. These events can result in either of two skin disorders, eczema or hives. Eczema is a red, scaly rash that commonly occurs in body folds such as in front of the elbow, behind the knee, and around the groin. It can usually be controlled with oral antihistamines and cortisone creams. Hives, also called urticaria, are red, raised, weltlike lesions on the skin, often occurring on the face and neck. Hives are often triggered by ingestion or inhalation of a substance, such as a medication or food, to which a person is allergic. Therefore, the most effective way of controlling hives is to recognize and avoid future contact with the allergen. Occasionally, hives may be an indication of a more serious, even life-threatening allergic reaction. In such situations, the airways can become constricted, making breathing difficult, and rapid intervention with injections of adrenaline may be required.

Bacteria

Bacteria is a one-celled organisms visible only through a microscope. Bacteria live all around us and within us. The air is filled with bacteria, and they have even entered outer space in spacecraft. Bacteria live in the deepest parts of the ocean and deep within Earth. They are in the soil, in our food, and on plants and animals. Even our bodies are home to many different kinds of bacteria. Our lives are closely intertwined with theirs, and the health of our planet depends very much on their activities.

Bacterial cells are so small that scientists measure them in units called micrometers (µm). One micrometer equals a millionth of a meter (0.0000001 m or about 0.000039 in), and an average bacterium is about one micrometer long. Hundreds of thousands of bacteria would fit on a rounded dot made by a pencil.

Bacteria lack a true nucleus, a feature that distinguishes them from plant and animal cells. In plants and animals the saclike nucleus carries genetic material in the form of deoxyribonucleic acid (DNA). Bacteria also have DNA but it floats within the cell, usually in a loop or coil. A tough but resilient protective shell surrounds the bacterial cell.

Biologists classify all life forms as either prokaryotes or eukaryotes. Prokaryotes are simple, single-celled organisms like bacteria. They lack a defined nucleus of the sort found in plant and animal cells. More complex organisms, including all plants and animals, whose cells have a nucleus, belong to the group called eukaryotes. The word prokaryote comes from Greek words meaning “before nucleus”; eukaryote comes from Greek words for “true nucleus.”

Immune System

Immune System, group of cells, molecules, and organs that act together to defend the body against foreign invaders that may cause disease, such as bacteria, viruses, and fungi. The health of the body is dependent on the immune system’s ability to recognize and then repel or destroy these invaders.

IMMUNITY

Most animals have systems that resist disease. The disease resistance provided by these systems is called immunity. There are two types of immunity: innate and adaptive. Innate, or nonspecific, immunity is the body’s first, generalized line of defense against all invaders. Innate immunity is furnished by barriers such as skin, tears, mucus, and saliva, as well as by the rapid inflammation of tissues that takes place shortly after injury or infection. These innate immune mechanisms hinder the entrance and spread of disease but can rarely prevent disease completely.

COMPONENTS OF THE IMMUNE SYSTEM

The ability of the immune system to mount a response to disease is dependent on many complex interactions between the components of the immune system and the antigens on the invading pathogens, or disease-causing agents.

A. Macrophage, type of white blood cell that resides in the tissues, out of the bloodstream, where it attacks foreign substances and helps the rest of the immune system identify harmful toxins, bacteria, and viruses.

B. Lymphocyte, variety of white blood cell consisting of T cells and B cells, both critical to the action of the immune system. T cells directly attack disease-causing bacteria, viruses, and toxins, and regulate the other parts of the immune system. B cells produce antibodies, which neutralize invaders or mark them for destruction by other agents of the immune system.

C. Antigen Receptors, one of the characteristics of adaptive immunity is that it is specific: Each response is tailored to a specific type of invading antigen. Each lymphocyte, as it matures, makes an antigen receptor—that is, a specific structure on its surface that can bind with a matching structure on the antigen like a lock and key. Although lymphocytes can make billions of different kinds of antigen receptors, each individual lymphocyte makes only one kind. When an antigen enters the body, it activates only the lymphocytes whose receptors match up with it.

D. Antigen-Presenting Cells - When an antigen enters a body cell, certain transport molecules within the cell attach themselves to the antigen and transport it to the surface of the cell, where they “present” the antigen to T lymphocytes. These transport molecules are made by a group of genes called the major histocompatibility complex (MHC) and are therefore known as MHC molecules. Some MHC molecules, called class I MHC molecules, present antigens to killer T cells; other MHC molecules, called class II MHC molecules, present antigens to helper T cells.

Septicemia

Septicemia, condition of the blood, characterized by increasingly high levels of bacteria in the circulatory system, with resulting infection of tissues and organs. The term sepsis generally refers to the source of infection, but is sometimes also used to refer to the condition of septicemia. Bacteremia (blood poisoning), the presence of bacteria in the blood, precedes the onset of septicemia and, unlike septicemia, is not life-threatening. Bacteremia is often asymptomatic, and a problem may not be apparent until the development of septicemia. In septicemia, bacteria may enter the bloodstream from an infected area of the body or after an injury or surgical procedure. Once in the bloodstream, the bacteria begin to multiply rapidly, spreading toxins throughout the circulatory system. Septicemia is characterized by chills, fever, tachycardia (rapid heart rate), tachypnea (rapid breathing), and a high white-blood-cell count. If septicemia is left untreated, septic shock, or sepsis syndrome, may ensue, a potentially fatal condition characterized by a dramatic drop in blood pressure, and damage to or failure of various organs, particularly the kidneys, heart, and lungs. Septicemia is more likely to occur in people who suffer from immunodeficiency (see Immune System). The condition is treated with appropriate antibiotics. See also Toxemia; Toxic Shock Syndrome.

Pneumonia

Pneumonia, inflammation of one or both lungs. In people with pneumonia, air sacs in the lungs fill with fluid, preventing oxygen from reaching blood cells and nourishing the other cells of the body. Sometimes the inflammation occurs in scattered patches in the tissue around the ends of the bronchioles, the smallest air tubes in the lungs. This is known as bronchopneumonia. In other cases the inflammation is widespread and involves an entire lobe of the lung. This condition is called lobar pneumonia. In the United States about 5 million cases of pneumonia are reported each year and about 63,500 people die from the disease.

Most cases of pneumonia result from infection with microorganisms, primarily viruses, bacteria, mycoplasmas (small, free-living particles with characteristics of both bacteria and viruses), and fungi. Pneumonia may also result from certain kinds of allergic reactions, inhalation of fluids or some gases, and the inhalation of ingested foods.

To determine the cause of pneumonia, a physician takes a sample of the patient's sputum. Analysis of the sputum in the laboratory may identify the particular kind of microorganism causing the infection. Identification of the cause of pneumonia is important in determining treatment.

Antibiotics can cure bacterial pneumonia and speed recovery from mycoplasma pneumonia and PCP. Antibiotics rarely have an effect on pneumonia caused by viruses. However, patients with viral pneumonia often receive antibiotics to prevent bacterial pneumonia from developing during the course of their illness. In addition to drug treatment, a patient with pneumonia should stay in bed, eat healthy meals, and drink large amounts of liquids. Medication may be given to relieve chest pain and violent coughing, and oxygen may be administered if the patient has difficulty breathing. A vaccine is available that confers immunity against pneumococcus. The vaccine is given to people most at risk for developing pneumonia—those over the age of 65 and those with chronic heart, lung, or liver disease.