These are of two types: viral neutralization tests and toxin neutralization tests.
1. Viral Neutralization
Neutralization of viruses by their antibodies can be demonstrated in various systems. IgG, IgM, and IgA antibodies can bind to some viruses during their extracellular phase and inactivate them. This antibody-mediated viral inactivation is called viral neutralization. Fixation of the classical pathway complement component C3b to the virus aids the neutralization process. Viral neutralization prevents a viral infection due to the inability of the virus to bind to its target cell.
Laboratory animals or tissue culture cells are used as “indicator systems” in these tests. The test serum is
diluted serially, incubated with a known amount of virus and the mixture is then added to indicator cultures: animals, embryonated hen’s egg and tissue culture.
The highest dilution of serum ablating infectivity in 50 per cent of virus-serum mixtures tested is taken as the titer. When bacteriophages are seeded in appropriate dilution on lawn cultures, plaques of lysis are produced. Specific antiphase serum inhibits plaque formation.
2. Toxin Neutralization
Bacterial exotoxins are good antigens and their activity may be completely neutralized by appropriate concentrations of specific antibody. Antibody to bacterial exotoxin is usually referred to as antitoxin which are important clinically, in protection against and recovery from diseases such as diphtheria and tetanus. Bacterial endotoxins are poorly antigenic and their toxicity is not neutralized by antisera.
Toxin neutralization can be tested in vivo or in vitro.
Toxin Neutralization In Vivo
i. Toxigenicity test: The neutralizing capacity of an antitoxin can be assayed by neutralization test in
which mixture of toxin and antitoxin is injected into a susceptible animal and the least amount of antitoxin that prevents death or disease in the animal is estimated.
ii. Schick test: With the diphtheria toxin, which in small doses causes a cutaneous reaction, neutralization test can be carried out on the human skin. The
Schick test is based on the ability to circulate antitoxin to neutralize the diphtheria toxin given intradermally. Neutralization (no reaction) indicates immunity and redness and erythema indicates susceptibility to diphtheria.
Toxin Neutralization In Vitro
If a toxin has a demonstrable in vitro effect, this effect can be neutralized by specific antitoxin.
i. Antistreptolysin O (ASO) test: Antistreptolysin O (ASO)—antitoxin, present in the serum of the patient suffering from Strep.pyogenes infection, neutralizes the hemolytic activity of the streptococcal O hemolysin (toxin).
ii. Nagler’s reaction: Another example of in vitro toxin-antitoxin neutralization is Nagler’s reaction. Cl.perfringens produces α-toxin which is a phospholipase (lecithinase-C). This produces opalescence in
serum or egg yolk media. This reaction is specifically neutralized by the antitoxin.
iii. Agar gel precipitation test: Agar gel precipitation test is employed to detect the production of toxin by Corynebacterium diphtheriae.