13.2D: Neutralization of Exotoxins

Learning Objectives

• Discuss how antibodies defend the body by way of neutralizing exotoxins. (Include what classes or isotypes of immunoglobulins are involved, the role of the Fab portion of the antibody, the role, if any, of the Fc portion of the antibody, and the role of any complement proteins, if any, involved.)
• Describe how the ability of bacteria to sense their own population density, communicate with each other by way of secreted factors (cell-to-cell signaling), and behave as a population rather than as individual bacteria most likely plays an important role in pathogenicity for many bacteria.

For an exotoxin to cause harm it must first bind to receptors on a susceptible host cell. Antitoxin antibodies are made against microbial exotoxins. The Fab portion binds to the exotoxin molecules before they can interact with host target cells and thus neutralizes the toxin (Figure \(\PageIndex{1}\)). IgG neutralizes toxins in tissues while IgA neutralizes toxins at mucosal surfaces within the body.

Figure \(\PageIndex{1}\): Neutralization of Exotoxins. The Fab portion of the antibodies made against epitopes of the binding site of an exotoxin blocks the exotoxin from binding to the exotoxin receptor on the host cell membrane. As a result, the toxin can not enter the cell and cause harm.

However, as learned in Unit 2, many Gram-negative and Gram-positive are able to sense their own population density, communicate with each other by way of secreted factors, and behave as a population rather than as individual bacteria. This is referred to as quorum sensing and most likely plays an important role in pathogenicity for many bacteria.

• For example, Pseudomonas aeruginosa causes severe nosocomial infections, chronic infections in people with cystic fibrosis, and potentially fatal infections in those who are immunocompromised. Its virulence depends on the secretion of a variety of harmful exotoxins and enzymes. If there was an isolated production of these virulence toxins and enzymes by a small number of Pseudomonas, the body's immune responses would most likely be able effectively neutralize these harmful agents with antibodies. However, through a coordination of the expression of the genes coding for these toxins and enzymes by the entire population of bacteria, P. aeruginosa appears to only secrete these extracellular virulence factors when the density of bacteria is large enough that they can be produced at high enough levels to overcome body defenses.

Summary

For an exotoxin to cause harm it must first bind to receptors on a susceptible host cell. Antitoxin antibodies are made against microbial exotoxins. The Fab portion binds to the exotoxin molecules before they can interact with host target cells and thus neutralizes the toxin.

Questions

Study the material in this section and then write out the answers to these questions. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.

1. Discuss how antibodies defend the body by way of neutralizing exotoxins. (Include what classes or isotypes of immunoglobulins are involved, the role of the Fab portion of the antibody, the role, if any, of the Fc portion of the antibody, and the role of any complement proteins, if any, involved.) (ans)
2. Describe how the ability of Pseudomonas aeruginosa to sense its own population density, communicate with other Pseudomonas by way of secreted factors (cell-to-cell signaling), and behave as a population rather than as individual bacteria most likely plays an important role in pathogenicity of this organism. (ans)
3. Multiple Choice (ans)