<unk>  refers to undesirable (damaging, discomfort-producing and sometimes fatal) reactions produced by the normal immune system.<unk>  reactions require a pre-sensitized (immune) state of the host. The four-group classification was expounded by P. H. G. Gell and Robin Coombs in 1963.
Hypersensitivity
Allergy
Cross-reactivity
Desensitization 
Ascending paralysis
Allergy pathophysiology
Radiosensitivity
Symbiosis