Q. How different are the actions of antibodies against virus and against bacteria? Why is the cellular immune response activated in case of chronic viral infection?
The antibodies of the humoral immune system act against extracellular agents, like bacteria or toxins but they are not active in the intracellular space and they cannot fight virus efficiently.
In case of viral infection (and also of precancerous or cancerous cells) the immune attack is made by the cellular immune system, mediated by NK (natural killers) and T, lymphocytes that destroy specific virus and cells.