3.7: Membrane surfaces have a “fingerprint” that identifies the cell.

Every cell in your body has a “fingerprint” made from a variety of molecules on the outside-facing surface of the cell membrane. Some of these membrane molecules differ from cell to cell, depending on the specific function of the cell. Others are common to all of your cells and tell your immune system, “I belong here.” Cells with an improper fingerprint are recognized as foreign and are attacked by your body’s defenses.

Throughout our evolutionary history, this system has been tremendously valuable in helping our bodies fight infection. In some cases, however, this vigilance is a problem, like a car alarm that goes off even when you don’t want it to. Suppose you receive a liver (or any other organ) transplant. Even if the donor is a close relative, the molecular fingerprint on the cells of the donated liver is not identical to your own. Consequently, your body sees the new organ as a foreign object and puts up a fight against it (FIGURE 3-15). Because your body will naturally try to reject the new organ, doctors must administer drugs that suppress your immune system. Immune suppression helps you tolerate the new liver, but, as you can imagine, it leaves you without some of the defenses to fight off other foreign invaders, such as bacteria that may cause infection.

The AIDS-causing virus, HIV, uses the molecular fingerprints on plasma membranes to infect an individual’s cells. These same molecular markers are also the reason that it’s extremely unlikely that you can catch an HIV infection from casual contact with an infected individual, such as shaking his or her hand. The specific molecular markers involved in infection by HIV belong to a group of identifying markers called “clusters of differentiation.” Abbreviated as “CD markers” and having names such as CD1, CD2, and CD3, these marker molecules are proteins embedded in the plasma membrane that enable the cell to bind to outside molecules and, sometimes, transport them into the cell.

One CD marker, called the CD4 marker, is found only on cells deep within the body and in the bloodstream, such as immune system cells and some nerve cells. It is the CD4 marker, in conjunction with another receptor, that is targeted by HIV. If the virus can find a cell with a CD4 marker, it can infect that cell, and because the CD4 markers never occur on the surface of skin cells, casual contact such as touching is very unlikely to transmit the virus (FIGURE 3-16). Even if millions of HIV particles are present on a person’s hands, they just can’t gain access into any of the other person’s surface cells.

The most common ways, by far, of transmitting HIV involve the transfer of blood, semen, vaginal fluid, or breast milk from one individual to another. Particles of the virus as well as cells infected by the virus are present in these fluids. Consequently, the chief routes of transmission are from an infected mother to her child in breast milk, from an infected mother to her baby at birth, the use of contaminated needles, and unprotected sexual intercourse. Because an open sore or cut might expose some of the cells in your bloodstream to the outside world, however, it is not impossible for casual transmission of HIV to occur.