Genetic Engineering and Human Health

Viruses

What is a virus, and how does it work?

A virus is a piece of genetic material (DNA or RNA) that is enclosed in a protective coat usually referred to as a capsid. It is the simplest forms of life, so simple, in fact, that there is some debate over whether it is a life form at all. It is believed that most viruses which infect humans evolved relatively recently, in the past five to ten thousand years, possibly as a consequence of our increasingly rapid evolution. ¹¹ Viruses are basically parasites which have 'learned' to hijack a cell. Once inside, they use the cell's own machinery to reproduce, assemble, package, and protect the newly produced viruses, which can survive outside the cell, and thereby propagate infection to other cells.

There are now about fifty different viruses that cause disease in humans. ¹² In contrast to bacteria, some of which are beneficial or even essential for our survival, a virus which contributes any positive health benefit at all has never been discovered. To have survived successfully, a virus needs a method of entry into its 'host' cell, and the ability to replicate once it gets there. It must also develop a way to thwart the host's natural defenses until it can establish an infection. Finally, the virus must figure out a means of transmission from one host to another; in short, to get out while the getting is good.

What happens when a virus enters a human host?

Humans have several types of defenses against viruses. The initial defense is the physical barriers we are born with; our skin, protective cells in our respiratory and reproductive tracts, and acidic conditions and destructive enzymes in our digestive tract. Because of the enormous number of viruses in existence, this defense is likely to be overwhelmed at some point.

The next defense in line is known as the innate defense system. It includes the phagocytes, which function as 'garbage eaters', and the neutrophils, a white blood cell whose job is to kill foreign agents that present in the blood. There are also about twenty different proteins that bind to the surface of any of these foreign objects, and tag them for destruction by one of these killer cells.

When the number of viruses overwhelms this system, the adaptive immune system comes into play. This involves several other types of white blood cells; B cells, killer T cells, T helper cells, and dendritic cells. B cells manufacture antibodies on demand for the purpose of identifying and tagging foreign invaders outside of the cell, much like the proteins mentioned above. The killer T cells, by contrast, have the ability to recognize what is happening inside of the cell, by scanning proteins on the cell surface. If these proteins exhibit properties of viral proteins, the killer T cell destroys the cell and the virus within.

Any one killer T cell has the ability to detect only one protein, but they are differentiated such that, together, their sum total can recognize any viral protein. However, the system needs time to manufacture enough of the particular killer T cells that are specific to the recognized viral protein, in the same way that the B cell manufactures antibodies. These processes can take up to a week or more. Unfortunately, this delay is what gives the virus time to establish itself in the host cell, and it is a critical factor for its survival.

Supervising both these operations of the adaptive immune system are the dendritic cells and the T helper cells, which might be thought of as messengers of this system, carrying information which orchestrates the entire process. Dendritic cells move to the lymph nodes, sometimes with the virus to be identified in tow, activating the T helper cells, which, in turn, assist in the activation of B cells and killer T cells. Some of these B and killer T cells will remain in the host as 'memory' cells, meaning they are always present, and ready to mount a quick defense against any subsequent viral infection that they 'remember'.

Finally, there is the interferon defense system. Human cells have sensor molecules which can detect a viral footprint, and turn on a signal which begins production of proteins called type 1 interferon. Interferon works within the cell to chemically halt viral reproduction, often killing the cell along with the virus inside it. It also moves out of the infected cell and warns other cells in the vicinity that they may be in danger of infection. Every known virus activates this defense system in the human body, and the most successful viruses have developed very clever ways to evade it. ¹³