THE
ADAPTIVE IMMUNE SYSTEM
II.
HUMORAL IMMUNITY
B.
WAYS THAT ANTIBODIES HELP TO DEFEND THE BODY
5.
Neutralization of Exotoxins
Fundamental Statements for this Learning Object:
1.For an exotoxin to cause harm it must first bind to receptors on a susceptible host cell.
2.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.
LEARNING
OBJECTIVES FOR THIS SECTION
Humoral Immunity refers to
the production of antibody molecules in response to an antigen (def).
These antibody molecules circulate in the plasma of the blood and enter tissue and organs via the inflammatory response.
Humoral immunity is most effective microbes or their toxins located in the extracellular spaces of the body.
Antibodies or immunoglobulins
(def)
are specific glycoprotein configurations produced by B-lymphocytes and plasma
cells in response to a specific antigen and capable of reacting with that
antigen.
The antibodies produced during humoral
immunity ultimately defend the body through a variety of different means. These
include:
1. Opsonization
2. MAC Cytolysis
3. Antibody-dependent Cellular Cytotoxicity (ADCC) by NK Cells
4. Neutralization of Exotoxins
5. Neutralization of Viruses
6. Preventing Bacterial Adherence to Host Cells
7. Agglutination of Microorganisms
8. Immobilization of Bacteria and Protozoans
9. Promoting an Inflammatory Response
In this section we will look at neutralization
of exotoxins.
5.
Neutralization of Exotoxins
For an exotoxin to cause harm
it must first bind to receptors on a susceptible host cell.
Antitoxin antibodies are made against microbial exotoxins (def).
The Fab portion binds to the exotoxin molecules before they can interact with host
target cells and thus neutralizes the toxin (see
Fig. 1). IgG neutralizes toxins in tissues while IgA neutralizes toxins at mucosal surfaces within the body.
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.
Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.
Last updated: Feb., 2020
Please send comments and inquiries to Dr.
Gary Kaiser