What is Anaphylaxis refers to the: and explain in detail?
Prince of Monaco first observed the deleterious effects of jellyfish on bathers. 
Subsequently, Portier and Richet (1906) suggested a toxin to be responsible for these effects and coined the term “anaphylaxis”.

Initiator cells in anaphylaxis
The initiator of type I reaction is otherwise known as allergen.
Typical allergens include:

Plant pollen
Proteins (e.g., foreign serum and vaccines)
Certain food items (e.g., eggs, milk, seafood, and nuts)
Drugs (e.g., penicillin and local anesthetics)
Insect products (venom from bees, wasps, and ants)
Dust mites and mould spores
Animal hair 
Dander

Parija SC. Textbook of Microbiology & Immunology. Elsevier Health Sciences; 2014. Page:149-150