by PhilipJ on 2 April 2006
Influenza virus is a dangerous enemy. Normally, the immune system fights off infections, eradicating the viruses and causing a few days of miserable flu symptoms. Yearly flu vaccines prime our immune system, making it ready to fight the most common strains of influenza virus. But once every couple of decades, and new strain of influenza appears that is far more pathogenic, allowing it to spread rapidly. This happened at the end of World War I, and the resultant pandemic killed over 20 million people, more than twice the number of people that were killed in the war.
Hemagglutinin is one of the reasons that influenza virus is so effective. It is a spike-shaped protein that extends from the surface of the virus. In the active form, shown here from PDB entry 1ruz, hemagglutinin is composed of two different types of chains, shown in blue and orange. The blue chains are the targeting mechanism: they search for specific sugar chains on our cellular proteins. When they find the proper one, hemagglutinin binds to the cell and the orange chains initiate the attack, as shown on the next page. The name hemagglutinin refers to the ability of influenza to agglutinate red blood cells: the virus is covered with many hemagglutinin molecules, which together can glue many red blood cells together into a visible clump.
As always, David Goodsell has high resolution images and more details here.