Exotoxins are heat labile proteins which are secreted by certain species of bacteria and diffuse readily into surrounding medium. They are highly potent in minute amount and constitute some of the most poisonous substances known.
One mg of tetanus or botulinum toxin is sufficient to kill more than one million guinea pigs and it has been estimated that 3 kg of botulinum toxin can kill all the inhabitants of the world.
Treatment of exotoxins with formaldehyde yield toxoids which are nontoxic but retain the ability to induce antibodies (antitoxins).
Exotoxins are generally formed by Gram positive bacteria but may also be produced by some Gram negative bacteria such as Shiga’s dysentery bacillus, vibrio cholera and enterotoxigenic E. coli.
Endotoxins are heat stable lipopolysaccharides (LPS) which form and integral part of the cell well of Gram negative bacteria.
Their toxicity depend on the lipid component (lipid A). they are not secreted outside the bacterial cell and are released only by the disintegration of the cell wall.
Endotoxins cannot be toxoided. They are poor antigens and their toxicity is not completely neutralized by the homologous antibodies. They are active only in relatively large doses.
All endotoxins, whether isolated from pathogenic or non pathogenic bacteria, produce similar effects. Administration of small quantities of endotoxin in susceptible animals causes an elevation of body temperature manifested within 15 minutes and lasting for several hours.
Intravenous injections of large doses of endotoxin and massive Gram negative septicemias cause endotoxin shock marked by fever, leucopenia, thrombocytopenia, significant fall in blood pressure, circulatory collapse and bloody diarrhea leading to death.