Running nose
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Vaccines Antibodies are our major defense against these small, efficient viruses. Vaccines prime the immune system with antibodies, making it ready to fight an infection. In the case of poliovirus, there are two types of vaccines. One is a killed version of the virus, which is slowly killed with formaldehyde over the course of several days so that it is inactivated, but still keeps its proper shape. The second is a weakened, but still live, strain of the virus that has been artificially bred to stimulate the immune system without causing disease. The immune system responds by making antibodies to fight these weakened viruses, leaving it ready to fight the real thing when it comes along. The polio vaccines are one of the triumphs of modern medicine, but many people would say that the lack of a cure for the common cold is one of the great failings. The difficulty of creating a vaccine for the common cold lies in the diversity of rhinovirus. Over one hundred types of rhinovirus have been discovered as they strike people around the world, and new strains appear continually. Rhinovirus is a moving target that is not effectively combated with a single vaccine. Antiviral drugs, however, are a possible solution. Picornavirus Structure Many viruses, including the picornaviruses and bacteriophage phiX174 (discussed in an earlier Molecule of the Month), are icosahedral in shape. They are composed of 60 identical pieces that form a perfectly symmetrical shell, termed a capsid, around the viral genome. In the case of poliovirus and rhinovirus, the shell is composed of 60 copies of four different proteins (colored yellow, orange, red, and magenta on rhinovirus here, PDB entry 4rhv), for a total of 240 protein chains in all. Notice that the fourth chain, colored magenta, can only be seen on the inside surface of the capsid. These proteins are carefully designed to be stable, but not too stable. They must be fairly sturdy to allow the virus to pass from host to host through the hostile environment. But at the same time, they must be able to fall apart when they enter the cell, releasing the RNA inside. A carefully orchestrated set of structural changes occur as the virus attaches to the surface of the cell and is drawn inside, allowing the virus to deliver its RNA into the unwitting host.
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