Final answer:
Viral specificity for a host is determined by the virus' surface glycoproteins that act as keys to the receptor lock on host cells, determining the virus' ability to infect specific species and cell types.
Step-by-step explanation:
Viral specificity for a certain host is determined by the virus' surface glycoproteins or attachment proteins. These proteins are like keys that fit into very specific locks—the receptor sites on the host cell membrane. Only a virus with the correct glycoprotein key can attach to and enter a cell with the corresponding receptor lock. This interaction dictates which species a virus can infect and even which cells within that species are vulnerable to infection.
For example, the influenza virus has specific glycoproteins that recognize and bind to receptors on cells of the respiratory system in humans. Similarly, the HIV virus uses glycoproteins such as gp120 in conjunction with CD4 and chemokine receptors to infect immune system cells. These are examples of how viral adhesins interact with cell receptors to define their tropism for certain types of cells and tissues.