Answer to Question 1
Whereas internal validity refers to how well the study establishes causal relationships, external validity refers to the generalizability of the results. High external validity means the study's findings generalize well to populations that have characteristics similar to the sample. Results of a research study that uses a large human sample tells us more about humans in general than results of a similar study with a small human sample or an animal study. For that reason, all other things being equal, the large human sample study is said to have higher external validity to human populations.
One study cannot match the potential generalizability of multiple studies statistically averaged together. When we average the results of a number of similar single studies of high methodological quality we can maximize both internal validity and external validity. It is important to note that if the studies being averaged have poor methodological quality, however, pooling them will not help.
Answer to Question 2
Cellular immunity cytokines stimulate other immune general responses as well, such as inflammation, fever, and wound healing. These chemical messenger molecules sometimes thought of as hormones of the immune system, are involved in mediating the inflammatory response. The homeostatic balance of the immune system is in large part dependent on the cytokines since some are pro-inflammatory (e.g., interleukin-6) and others are anti-inflammatory (e.g., interleukin-4). Cytokines include interleukin, interferon, and tumor necrosis factor.
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