@A.B.
Agreed, just looking at the key serum contents, one thing stands out to me:
Antigens
I'm of the opinion that it is some kind of large antigen/molecule which triggers the initial insult/response and then on a very basic level the antibody/antibodies produced as a result of this antigen mean that every time we exert ourselves we trigger an immune response/cascade of reactions which leave us feeling terrible. All speculation of course, but that is my educated guess.
The antigens/molecules which can trigger immune responses and activate lymphocytes are called immunogens apparently.
>> EDIT: On reflection my comment about antigens is probably way off. I'm gunna leave it on here but it could be complete nonsense so i'd probably ignore it. As many have already said, best to leave it to the experts