Which feature is NOT characteristic of IgA?

Immunoglobulin A (IgA) is primarily known for its role in mucosal immunity and is commonly found in external secretions such as saliva, tears, and breast milk, as well as in the bloodstream. Its primary function is to provide a first line of defense against pathogens at mucosal surfaces. This includes areas such as the respiratory and gastrointestinal tracts, where IgA acts to neutralize toxins and prevent the adhesion of pathogens to epithelial cells.

The structure of IgA is typically dimeric, consisting of two IgA monomers joined by a J chain, rather than pentameric. The pentamer form is characteristic of Immunoglobulin M (IgM), which has a different role in the immune response. While IgA does indeed play a significant role in defending against pathogens, it is the specific dimeric structure that distinguishes it from IgM's pentameric form. Thus, the pentamer structure is not a feature characteristic of IgA.