Which type of immunity involves a direct response to viral infections?

Cell-mediated immunity is the correct choice because it specifically involves the activation of T cells, which play a central role in responding to viral infections. When a virus infects a host cell, the infected cells present viral antigens on their surface. Cytotoxic T cells recognize these antigens through their T cell receptors and initiate a direct immune response. This response involves the destruction of the virus-infected cells, which helps to limit the spread of the virus within the host.

In contrast, humoral immunity primarily involves the production of antibodies by B cells, which are more effective against extracellular pathogens rather than directly targeting virus-infected cells. Innate immunity provides the first line of defense through non-specific mechanisms, such as physical barriers and innate immune cells, but it does not involve the specific recognition of viral antigens like cell-mediated immunity does. Passive immunity entails the transfer of antibodies from one individual to another, providing temporary protection but not engaging the recipient's immune system to actively respond to infections. Thus, for a direct response to viral infections, cell-mediated immunity is essential.