ADE can arise in several different ways but the best-known is dubbed the ‘Trojan Horse Pathway’. This occurs when non-neutralizing antibodies generated by past infection or vaccination fail to shut down the pathogen upon re-exposure.
Instead, they act as a gateway by allowing the virus to gain entry and replicate in cells that are usually off limits (typically immune cells, like macrophages). That, in turn, can lead to wider dissemination of illness, and over-reactive immune responses that cause more severe illness.
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Good, short explanation of Antibody Dependent Enhancement (ADE)
American Buddhist Net 