Viruses need to get their genome inside of a cell in order to replicate. They use specific proteins on their surface, called antigens, to bind to proteins, called receptors, on the surface of the cell to gain entrance. Antigens are like a key that must fit exactly into the corresponding lock on the cell surface.
In the image on the left it is obvious that virus A can only bind to cell A because its antigen can only fit into the receptor on cell A. For the same reason, virus B can only bind and infect cell B.
Antibodies are depicted as a "Y" structure with three arms that are roughly of equal size, joined by a flexible hinge. The red area shown in the illustration is called the constant region. Depending on the type of antibody it does not vary in structure. The blue regions shown in the illustration are called variable regions. The conformation varies depending on the antigen they are designed to bind to.
In this illustration an antibody (shown at an exaggerated size) has been designed by the immune system to fit the shape of the antigen of virus A. When the antibody binds to the antigen this prevents the virus from attaching to the surface receptor on the cell.
Virus A is neutralized by the antibody since it cannot infect cell A. Virus B is still capable of binding to the receptor B and infecting the cell. The constant region of the antibody bound to virus A will signal other immune cells to come and destroy virus A.
Even though viruses frequently alter their surface antigens in an effort to evade the antibody immune response, this system works very well for most viral infections.
Antibody neutralization of the CAE virus is an exception.
Next: CAE virus and the immune system