Viruses generally enter host cells primarily by binding to specific receptors on the cell surface. This process is crucial because viruses lack metabolic machinery and must rely on the host cell's machinery to replicate.
The binding occurs when viral proteins, often on their surface, interact specifically with receptor molecules on the host cell's membrane. This receptor-ligand interaction is highly specific, often determining the range of host cells that a virus can infect. For instance, the HIV virus binds to the CD4 receptor on T cells, which is a key step in its entry into the immune system.
Once the virus is attached to the receptor, various mechanisms can facilitate the actual entry into the cell, such as endocytosis or direct fusion with the membrane, but the initial and critical step is the binding to the surface receptors. This specificity not only influences the virus's infectivity but also its host range, which can vary significantly between different viruses depending on the receptors available in potential host cells.