Immunological Diseases

Immunological diseases occur when the immune system, which normally protects against infections, malfunctions. The most common types of immunological diseases are allergies, autoimmune diseases, and immune deficiencies.

An allergy is an abnormal reaction of the immune system to foreign substances, such as plant pollen, fungal spores, animal danders, medications, and foods. Rhus dermatitis is an allergy caused by contact with urushiol, an oil resin produced by poison ivy, poison oak, and poison sumac.

Autoimmune diseases develop when the immune system goes awry and attacks the body's own tissues. Autoimmune disorders includes lupus erythematosus, rheumatoid arthritis, juvenile-onset diabetes, and myasthenia gravis. The causes are unknown, although some scientists suspect the diseases may be triggered in some cases by a pathogen, such as a virus, or other environmental factor.

Immune deficiency diseases develop when the immune system becomes impaired, resulting in more common, frequent, or severe infections. The immune system may be damaged by a genetic abnormality or by illness, injury, the use of a strong drug such as those used in chemotherapy, or malnutrition.

Virus

Virus (life science), infectious agent found in virtually all life forms, including humans, animals, plants, fungi, and bacteria. Viruses consist of genetic material—either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)—surrounded by a protective coating of protein, called a capsid, with or without an outer lipid envelope. Viruses are between 20 and 100 times smaller than bacteria and hence are too small to be seen by light microscopy. Viruses vary in size from the largest poxviruses of about 450 nanometers (about 0.000014 in) in length to the smallest polioviruses of about 30 nanometers (about 0.000001 in). Viruses are not considered free-living, since they cannot reproduce outside of a living cell; they have evolved to transmit their genetic information from one cell to another for the purpose of replication.

Viruses often damage or kill the cells that they infect, causing disease in infected organisms. A few viruses stimulate cells to grow uncontrollably and produce cancers. Although many infectious diseases, such as the common cold, are caused by viruses, there are no cures for these illnesses. The difficulty in developing antiviral therapies stems from the large number of variant viruses that can cause the same disease, as well as the inability of drugs to disable a virus without disabling healthy cells. However, the development of antiviral agents is a major focus of current research, and the study of viruses has led to many discoveries important to human health.

Amino Acids

Amino Acids, important class of organic compounds that contain both the amino (8NH2) and carboxyl (8COOH) groups. Of these acids, 20 serve as the building blocks of proteins. Known as the standard, or alpha, amino acids, they comprise alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. All 20 are constructed according to a general formula:

As the formula shows, the amino and carboxyl groups are both attached to a single carbon atom, which is called the alpha carbon atom. Attached to the carbon atom is a variable group (R); it is in their R groups that the molecules of the 20 standard amino acids differ from one another. In the simplest of the acids, glycine, the R consists of a single hydrogen atom. Other amino acids have more complex R groups that contain carbon as well as hydrogen and may include oxygen, nitrogen, or sulfur, as well.

When a living cell makes protein, the carboxyl group of one amino acid is linked to the amino group of another to form a peptide bond. The carboxyl group of the second amino acid is similarly linked to the amino group of a third, and so on, until a long chain is produced. This chainlike molecule, which may contain from 50 to several hundred amino acid subunits, is called a polypeptide. A protein may be formed of a single polypeptide chain, or it may consist of several such chains held together by weak molecular bonds. Each protein is formed according to a precise set of instructions contained within the nucleic acid, which is the genetic material of the cell. These instructions determine which of the 20 standard amino acids are to be incorporated into the protein, and in what sequence. The R groups of the amino acid subunits determine the final shape of the protein and its chemical properties; an extraordinary variety of proteins can be produced from the same 20 subunits.

The standard amino acids serve as raw materials for the manufacture of many other cellular products, including hormones and pigments. In addition, several of these amino acids are key intermediates in cellular metabolism.

Most plants and microorganisms are able to use inorganic compounds to make all the amino acids they require for normal growth. Animals, however, must obtain some of the standard amino acids from their diet in order to survive; these particular amino acids are called essential. Essential amino acids for humans include lysine, tryptophan, valine, histidine, leucine, isoleucine, phenylalanine, threonine, methionine, and arginine. They are found in adequate amounts in protein-rich foods from animal sources or in carefully chosen combinations of plant proteins.

In addition to the amino acids that form proteins, more than 150 other amino acids have been found in nature, including some that have the carboxyl and amino groups attached to separate carbon atoms. These unusually structured amino acids are most often found in fungi and higher plants.

Pregnancy

Pregnancy starts when a male’s sperm fertilizes a female’s ovum (egg), and the fertilized ovum implants in the lining of the uterus (see Fertilization; Reproductive System). Because pregnancy changes a woman’s normal hormone patterns, one of the first signs of pregnancy is a missed menstrual period (see Menstruation). Other symptoms include breast tenderness and swelling, fatigue, nausea or sensitivity to smells, increased frequency of urination, mood swings, and weight gain. Some women also experience cravings for unusual substances such as ice, clay, or cornstarch; this condition, called pica, can indicate a dietary deficiency in iron or other nutrients. By the 12th week of pregnancy many of these symptoms have subsided, but others appear. For example, a woman’s breasts usually increase in size, and her nipples darken. The most obvious symptom is weight gain; most physicians now recommend a gain of about 9 to 12 kg (about 22 to 26 lb) by the end of pregnancy.

The first few months of pregnancy are the most critical for the developing infant, because during this period the infant’s brain, arms, legs, and internal organs are formed. For this reason a pregnant woman should be especially careful about taking any kind of medication except on the advice of a physician who knows that she is pregnant. X rays should also be avoided, and pregnant women should avoid smoking and alcohol consumption.

Hereditary and Congenital Diseases

Hereditary diseases such as hemophilia, sickle-cell anemia, Huntington's disease, muscular dystrophy, and Tay-Sachs disease are caused by mutated genes inherited from one or both parents (see Genetic Disorders). Certain other diseases, such as diabetes mellitus, hypertension, and some types of cancer, often run in families, which suggests that heredity is at least partially responsible for their development.

Congenital diseases, or birth defects, are disorders that are present at birth. Some are hereditary, others develop while a baby is in its mother's uterus or during the process of delivery. For example, if the mother contracts German measles, or rubella, during the early stages of pregnancy, her child may be born with heart defects, eye cataracts, deafness, or mental retardation. Use of alcohol during pregnancy can cause fetal alcohol syndrome, characterized by mental and physical retardation. Abnormal development of any body part in a fetus may produce a congenital defect; for example, if walls that separate the chambers of the heart fail to form completely, the baby is born with congenital heart disease.