host range

Meaning: Host range is the set of organisms a virus can infect and replicate in. The first step is attachment, in which a viral capsid or envelope protein binds a specific receptor on the host cell surface. If the host cell lacks the matching receptor, the virus usually cannot enter. After entry, the virus also needs compatible host machinery (ribosomes, enzymes, tRNAs) and must get past the cell's defenses. The cell-type version is called tissue tropism. HIV, for example, targets cells that carry CD4. Bacteriophages often have narrow host ranges, limited to certain bacterial species or strains.

On the MCAT: A passage may describe a virus that infects one species, then introduce a mutation in a surface protein or a change in a host receptor. You predict whether the host range widens or narrows. Mutation, or reassortment of segmented genomes as in influenza, can alter receptor binding and let a virus spill over to a new species. Another common setup mixes phages with different bacterial strains and asks which cultures lyse. A cell that lacks the receptor, or has a mutated one, is typically resistant.

Common confusion: Binding a receptor is necessary but does not guarantee a productive infection, because the cell must also support replication. A broad host range means many species can be infected. It does not mean every cell type in each of them can be.