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ertain people are naturally resistant to HIV, the virus that

C causes AIDS. For example, a woman received a blood transfusion In 1980 that was later found to be contaminated with HIV, but she never became Infected. Some Intravenous drug users share needles with people who later develop AIDS, and some prostitutes exposed to many HIV-posltlve men never themselves become Infected.

We usually think of avoiding AIDS by avoiding activities that spread the virus, and this is without doubt the best course. But what protects these people, all of whom have been exposed to HIV? A lucky few individuals cannot contract AIDS because of an abnormality in their cells.

When HIV enters a human body, it approaches certain white blood cells, called CD4 helper T cells, that control the immune system. The virus binds first to receptors called CD4—the receptors are proteins that extend from the cell surface. Once bound, HIV moves down the

CD4 receptor and binds another receptor, called CCR5. Only then can the virus enter the cell, and start the chain reaction of viral replication that ultimately topples immunity.

Thanks to heredity, one percent of Caucasians in the United States, and far fewer Asians, African Americans, and Native Americans, have cell surfaces that lack the crucial CCR5 HIV docking sites. These lucky few individuals cannot get AIDS, because HIV cannot enter their cells. Another 20% of the Caucasian population (less for others) have half the normal number of CCR5 receptors. These people can become infected, but remain healthy longer than is usual.

Researchers are now applying this knowledge of how AIDS begins at the cellular level to develop vaccines and new treatments. Understanding how HIV interacts with cells, the units of life, has revealed what might finally prove to be HIV's Achilles heel—a protein portal called CCR5.

An adult human body consists of about 75 trillion cells, the basic units of an organism. All cells have much in common, yet those in different tissues are distinctive in a number of ways.

Cells vary considerably in size. We measure cell sizes in units called micrometers (mickro-mecterz). A micrometer equals one thousandth of a millimeter and is symbolized ^m. A human egg cell is about 140 ^m in diameter and is just barely visible to an unaided eye. This is large when compared to a red blood cell, which is about 7.5 ^m in diameter, or the most common types of white blood cells, which vary from 10 to 12 ^m in diameter. On the other hand, smooth muscle cells can be between 20 and 500 ^m long (fig. 3.1).

Cells also vary in shape, and typically their shapes make possible their functions (fig. 3.2). For instance, nerve cells often have long, threadlike extensions many centimeters long that transmit nerve impulses from one part of the body to another. Epithelial cells that line the inside of the mouth are thin, flattened, and tightly packed, somewhat like floor tiles. They form a barrier that shields underlying tissue. Muscle cells, which contract and pull structures closer together, are slender and rodlike, with their ends attached to the parts they move. Muscle cells are filled with contractile proteins. An adipose cell is little more than a blob of fat; a B lymphocyte is an antibody factory. The human body is a conglomeration of many types of cells.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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