Final answer:
Host cell specificity is given by the interaction of viral adhesins with specific cell receptors on the host cell. Examples include the HIV glycoprotein gp120's binding to the CD4 molecule on T lymphocytes and the Influenzavirus spike protein hemagglutinin's binding to sialic acid on respiratory and intestinal cells.
Step-by-step explanation:
The part of the adhesion that gives host cell specificity is the interaction between viral adhesins, such as glycoproteins or proteins on the viral capsid or envelope, and the specific receptors on the host cell surface. These receptors are integral to the cell's normal functions but are exploited by viruses to facilitate attachment and entry into cells. For example, the glycoprotein gp120 on HIV binds to the CD4 molecule, a type of cell adhesion molecule, on T lymphocytes. This specificity is akin to a key fitting into a lock, where only the correct key (viral adhesin) will fit into the specific lock (host cell receptor).
Adhesins can be found on various infectious agents, including bacteria, protozoa, and viruses. The spike protein hemagglutinin on Influenzavirus and the glycoprotein gp120 on HIV are examples of adhesins that allow these viruses to attach to cells with corresponding receptors. Such mechanisms enable viruses to infect specific cell types within certain tissues and organs, determining which part of the body is affected by the viral infection.