25 year old with a wbc of 5.1. is this a cause for concern ? is it cancer ? what if it drops to 4.0 ? thank you

Depends on the story: Lab result numbers are just numbers and often have little meaning without the whole story. A patient and his doctor can discuss the story behind the lab result and make sense of it. Without the big picture, one can only say that normal people on a routine screening blood test often have the normal result of 4, 5, 6, or 7 thousand white blood cells.