[3][4] Neutralizing antibodies are part of the humoral response of the adaptive immune system against viruses, bacteria and microbial toxin.

[5] Neutralizing antibodies can inhibit infectivity by binding to the pathogen and blocking the molecules needed for cell entry.

This can be due to the antibodies statically interfering with the pathogens, or toxins attaching to host cell receptors.

Furthermore, neutralizing antibodies can act by preventing particles from undergoing structural changes often needed for successful cell entry.

For example, neutralizing antibodies can prevent conformational changes of viral proteins that mediate the membrane fusion needed for entry into the host cell.

When B cells are produced in the bone marrow, the genes that encode the antibodies undergo random genetic recombination (V(D)J recombination), which results in every mature B cell producing antibodies that differ in their amino acid sequence in the antigen-binding region.

[12] A strong diversity in the antibody repertoire allows the immune system to recognize a plethora of pathogens which can come in all different forms and sizes.

When the B-cell receptor binds to its cognate antigen with high affinity, an intracellular signalling cascade is triggered.

[13] After a first encounter of the antigen by vaccination or natural infection, immunological memory allows for a more rapid production of neutralizing antibodies following the next exposure to the virus.

Conversely, antibodies also simultaneously evolve by affinity maturation during the course of an immune response, thereby improving recognition of viral particles.

However, viruses have evolved certain mechanisms to hinder steric access of an antibody to these regions, making binding difficult.

[19][20] Serum therapy was shown to reduce mortality in patients during the 2009 swine flu pandemic[21] and the Western African Ebola virus epidemic.

[23][24] Immunoglobulin therapy, which uses a mixture of antibodies obtained from healthy people, is given to immunodeficient or immunosuppressed patients to fight off infections.

The antigen injected into the animal donors can be designed in such a way to preferably produce neutralizing antibodies.

[29][30] Introducing a weakened form of a virus through vaccination allows for the production of neutralizing antibodies by B cells.

[2] Although this type of antibody has the ability to fight retroviral infections, in some cases it attacks pharmaceuticals administered to the body which would otherwise treat multiple sclerosis.

[2] Neutralization assays are capable of being performed and measured in different ways, including the use of techniques such as plaque reduction (which compares counts of virus plaques in control wells with those in inoculated cultures), microneutralization (which is performed in microtiter plates filled with small amounts of sera), and colorimetric assays (which depend on biomarkers indicating metabolic inhibition of the virus).

[32] Most of the neutralizing antibodies produced by the immune system are very specific for a single virus strain due to affinity maturation by B cells.