Microbial evasion of IgA functions

This chapter focuses on microbial strategies specifically related to evasion of immunoglobulin A (IgA) functions, in particular in humans. IgA1 proteases are unique in their specific ability to cleave human IgA1. By attacking a single peptide bond in the IgA1 hinge region, IgA1 proteases eliminate all secondary effector functions of IgA1 antibodies while enabling the bacteria to take advantage of the released monomeric antigen-binding fragments for colonization and evasion of other parts of the immune system. Several lines of direct and indirect evidence indicate that the activity of IgA1 proteases secreted by bacteria colonizing the upper respiratory tract may result in a local functional IgA1 deficiency that facilitates penetration or alternative processing of potential allergens. However, because of the lack of a satisfactory animal model, full appreciation of the biological significance of IgA1 proteases must await successful strategies to prevent their activity, for example, by active or passive immunization. Further studies are necessary to understand how bacterial IgA binding proteins and glycosidases that attack the carbohydrate moiety of IgA may interfere with its protective functions.