Polymeric Immunoglobulin Receptor

This chapter discusses the cellular and molecular mechanisms involved in immunoglobulin A (IgA) transport across epithelial cells. The major class of immunoglobulin found in a wide variety of mucosal secretions, such as gastrointestinal and respiratory secretions, is IgA. IgA is produced by submucosal plasma cells that are often found in lymphoid aggregates, such as gut-associated lymphoid tissue and bronchus-associated lymphoid tissue. After secretion from the plasma cells, IgA is taken up by an overlying epithelial cell, transported across the epithelium, and released into external secretions. The IgA forms the first specific immunologic defense against infection. This transport system transports only polymeric immunoglobulins. Substantial progress has been made in understanding the cellular and molecular basis of the transcytosis of immunoglobulins. This knowledge is important for two reasons. First, transcytosis is one of several systems of protein traffic in epithelial cells. Analyzing the sorting signals and cellular machinery that decodes these signals will permit elucidation of the general principles that govern protein sorting. Second, transcytosis is important in the overall physiology of an organism.