Schematic model for cellular immunity to <i>Salmonella</i>.

<p><i>Salmonella</i> (yellow) reside in intracellular vacuoles in infected host cells. <i>Salmonella</i> possesses internal (green) and surface-associated (red) antigens. <b>Left</b>) Live <i>Salmonella</i> shield internal antigens, but some of their surface-associated antigens are accessible for processing and presentation. As a consequence, T cells specific for <i>Salmonella</i> surface antigens can recognize these infected cells and initiate antibacterial immune effector mechanisms. In contrast, T cells specific for internal <i>Salmonella</i> antigens fail to detect host cells that contain exclusively intact <i>Salmonella</i>. <b>Right</b>) Dead <i>Salmonella</i> release internal antigens. As a consequence, both surface-exposed and internal antigens can be processed, presented, and recognized by cognate T cells. However, this recognition is unproductive for infection control since it targets <i>Salmonella</i> that are already dead.