Nagler's  reaction is an type of
Cl. perfringens is grown on a medium containing 6% agar, 5% Fildes' peptic digest of sheep blood and 20% human serum or 5% egg yolk in a plate. 
Neomycin sulphate may be added to the medium to make it more selective by inhibiting aerobic spore bearers and coliforms. 
To one half of the plate, antitoxin is spread on the surface. 
The inoculated culture plate is incubated at 37°C for 24 hours. 
Colonies on the half plate without the antitoxin will be surrounded by opacity while colonies on the other half with antitoxin shows no opacity, due to specific neutralization of the alpha toxin.
Alpha toxin (lecithinase C) splits lecithin into phosphoryl choline and a diglyceride (lipid). 
The lipid deposits around the colonies resulting in opacity.

Ref : textbook of microbiology, C P Baveja 5th edition pg no:228
Neutralization reaction
Complement fixation test
Agglutination test
Precipitation reaction