Histidine is converted to histamine by:
Active form of Pyridoxine

Pyridoxal Phosphate (PLP)
Mainly used for Amino Acid metabolism.

Coenzyme Role of Pyridoxal Phosphate (PLP)
Transamination

Alanine Amino Transferase (ALT)
Aspartate Amino Transferase (AST)
Alanine Glyoxalate Amino Transferase.

Decarboxylation of amino acids
This results in the formation of Biogenic Amines

Glutamate: CABA
5-Hydroxy Tryptophan: Serotonin
Histidine: Histamine
Cysteine: Taurine
Serine: Ethanolamine
DOPA: Dopamine.

Key Concept:
Histidine is converted to Histamine by the process of decarboxylation.
Transamination
Hydroxylation
Decarboxylation
Reduction