Microbiology, 2021

810 20 • Laboratory Analysis of the Immune Response
MICRO CONNECTIONS
Using Monoclonal Antibodies to Combat Ebola
During the 2014–2015 Ebola outbreak in West Africa, a few Ebola-infected patients were treated with ZMapp, a
drug that had been shown to be effective in trials done in rhesus macaques only a few months before. 5 ZMapp
is a combination of three mAbs produced by incorporating the antibody genes into tobacco plants using a viral
vector. By using three mAbs, the drug is effective across multiple strains of the virus. Unfortunately, there was
only enough ZMapp to treat a tiny number of patients.
While the current technology is not adequate for producing large quantities of ZMapp, it does show that
plantibodies—plant-produced mAbs—are feasible for clinical use, potentially cost effective, and worth further
development. The last several years have seen an explosion in the number of new mAb-based drugs for the
treatment of cancer and infectious diseases; however, the widespread use of such drugs is currently inhibited
by their exorbitant cost, especially in underdeveloped parts of the world, where a single dose might cost more
than the patient’s lifetime income. Developing methods for cloning antibody genes into plants could reduce
costs dramatically.
20.2 Detecting Antigen-Antibody Complexes
Learning Objectives
By the end of this section, you will be able to:
• Describe various types of assays used to find antigen-antibody complexes
• Describe the circumstances under which antigen-antibody complexes precipitate out of solution
• Explain how antibodies in patient serum can be used to diagnose disease
Laboratory tests to detect antibodies and antigens outside of the body (e.g., in a test tube) are called in vitro
assays. When both antibodies and their corresponding antigens are present in a solution, we can often observe
a precipitation reaction in which large complexes (lattices) form and settle out of solution. In the next several
sections, we will discuss several common in vitro assays.
Precipitin Reactions
A visible antigen-antibody complex is called a precipitin, and in vitro assays that produce a precipitin are
called precipitin reactions. A precipitin reaction typically involves adding soluble antigens to a test tube
containing a solution of antibodies. Each antibody has two arms, each of which can bind to an epitope. When
an antibody binds to two antigens, the two antigens become bound together by the antibody. A lattice can form
as antibodies bind more and more antigens together, resulting in a precipitin (Figure 20.5). Most precipitin
tests use a polyclonal antiserum rather than monoclonal antibodies because polyclonal antibodies can bind to
multiple epitopes, making lattice formation more likely. Although mAbs may bind some antigens, the binding
will occur less often, making it much less likely that a visible precipitin will form.
5 Qiu, Xiangguo et al., “Reversion of Advanced Ebola Virus Disease in Nonhuman Primates with ZMapp,” Nature 514 (2014): 47–53.
Access for free at openstax.org.