TNF (alpha if you want) is a prime defense mechanism of the [wiki2="Immune-System"]immune system[/wiki2], it's a [wiki2="Cytokine"]cytokine[/wiki2] release by macrophages, depending on if the macrophage is empty or not and depending on the pathogen, the release can be low or extremely high.
The idea of some people is that by avoiding excessive TNF you can lower the immune response, and while this seems to be true, you are also lowering your primary defense mechanism.
TNF are extremely powerful and your body needs them to fight of infections and to destroy certain cells.
There's also a lot of arguments back and forth how important TNF is and how come crohn have high levels of TNF, if TNF is being released because of some immune response that is misguided or if it's simply being release from macrophages by "trojan horse" pathogens like AIEC or MAP.
When macrophages become infected with MAP and AIEC, the release of TNF is extremely high, but if it's high enough to explain the increased TNF in crohn's disease, they don't know.
Also, there's arguments about which is the main contributer to tissue damage, is it actually the immune response or is it related to oxidative stress (ROS) , which can be worse if TNF levels are higher and vice versa.
