found ( that the people in the group that watched the video 主语) had (谓语)an increase in the activity of their while blood cells,that is,the cells which fight infection
an increase 是宾语,
in the activity of their white blood cells,是地点状语
前面半句意思:发现观看视频的那组人,他们的白血球细胞的活跃性有了提升.
后面半句:that is the cells which fight infection 定语从句,修饰white blood cells.