What defines Cell-Mediated Immunity?

Cell-mediated immunity is defined primarily by the actions and functions of T cells, specifically the cytotoxic T cells, which play a crucial role in identifying and directly destroying infected or cancerous cells. This type of immune response is essential for dealing with intracellular pathogens, such as viruses, and is pivotal in situations where antibodies, which are produced by B cells, would not be effective.

In cell-mediated immunity, T cells recognize antigens presented on the surface of infected cells via Major Histocompatibility Complex (MHC) molecules. Once activated, these T cells can directly kill infected host cells, thereby inhibiting the spread of the pathogen. This direct interaction and elimination of compromised cells differentiate cell-mediated immunity from humoral immunity, where the primary role focuses on antibody production and neutralization of pathogens in extracellular spaces.

The other options relate more to mechanisms utilized in humoral immunity, which revolves around the production of antibodies by B cells. While there is a collaborative aspect where both arms of the immune system work together, cell-mediated immunity is fundamentally marked by the action of T cells executing cytotoxic functions.