Final answer:
Natural killer (NK) cells are the primary defense against viruses in a pediatric patient with a viral infection, as part of the innate immune response.
Step-by-step explanation:
In the context of a pediatric client admitted with a viral infection, the child's primary defense against the virus would be natural killer (NK) cells. These cells are a crucial part of the innate immune response and play a pivotal role in the body's first line of defense against viruses. NK cells can recognize and induce apoptosis, or programmed cell death, in infected cells by mechanisms that involve their surface receptors and the altered expression of major histocompatibility complex (MHC) I molecules on the surface of the infected cells. They work in concert with cytotoxic T cells to destroy virally infected cells.