Viral attachment to host entry receptor

Entry receptors trigger virus entry by endocytosis/pinocytosis or by inducing fusion/penetration. The consequences of this binding are irreversible. They have often be named "co-receptors". Entry receptors are often difficult to access for the virion, which circumvents this problem by binding first to adhesion receptors, which increases the probability of binding to the entry receptor.

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In herpesviruses for example, gC makes the primary contact with the host cell surface, binding to heparan sulfate proteoglycans. Binding of gD to the entry receptors induces viral fusion to the host cell membrane and viral penetration .
In bacterial viruses, attachment to the entry receptor is often referred to as irreversible adsorption, based on experimental observations .