THE
ADAPTIVE IMMUNE SYSTEM
II.
HUMORAL IMMUNITY
B.
WAYS THAT ANTIBODIES HELP TO DEFEND THE BODY
6.
Neutralization of Viruses
Fundamental Statements for this Learning Object:
1. In order for viruses to infect a cell and replicate, they must first adsorb to receptors on the host cell's plasma membrane.
2. Antibodies are made against viral capsids or envelope glycoproteins where the Fab portion binds to and covers the viral attachment molecules. This prevents viral adsorption to host cells.
3. Neutralizing antibodies are especially important in preventing viral reinfection.
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 viruses.
6. Neutralization
of Viruses
In order for viruses to infect
a cell and replicate, they must first adsorb to receptors on the host cell's
plasma membrane.
Antibodies are made against viral
capsids or envelope glycoproteins where the Fab portion binds to and covers the viral attachment molecules. This prevents viral adsorption to host cells. (see
Fig. 1). Neutralizing antibodies are especially important in preventing
viral reinfection. IgG neutralizes viruses in tissues while IgA neutralizes viruses at mucosal surfaces within the body.
However, as learned in Unit 4,
some viruses by means of the activities described below are able to overcome
this antibody defense.
- The influenza viruses undergo
what is called antigenic drift and antigenic shift. With antigenic
drift, mutations cause a gradual change in the hemagglutinin antigen that
adsorbs to receptors on host cells. Antigenic shift is caused by a human
influenza virus acquiring a new genome segment from an influenza virus capable
of infecting other animals such as a ducks or swine. This new genome segment
causes a major change in the hemagglutinin antigen. Antibodies made against
the original human influenza virus can no longer bind to the new strain
of virus or stick the virus to phagocytes.
- Likewise HIV, because of its
high rate of mutation and its intracellular recombination with other strains
of HIV, as mentioned earlier in this unit, produces altered gp120 to
which antibodies made against the earlier strains of HIV can no longer bind.
- The hepatitis C virus (HCV)
frequently, through mutation, produces viral variants ("escape mutants") to resist antibodies.
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