<P> Oral tolerance is the process by which the immune system is prevented from responding to antigen derived from food products, as peptides from food may pass into the bloodstream via the gut, which would in theory lead to an immune response . A paper published in Nature in 2012 has shed some light on the process and implicated goblet cells as having a role in the process . It was known that CD103 - expressing dendritic cells of the lamina propria had a role to play in the induction of oral tolerance (potentially by inducing the differentiation of regulatory T cells), and this paper suggests that the goblet cells act to preferentially deliver antigen to these CD103 dendritic cells . </P> <P> The excessive mucus production seen in allergic asthma patients is due to goblet cell metaplasia, the differentiation of airway epithelial cells into mucin producing goblet cells . These cells produce the thick mucins MUC5AC and MUC5B, which clog the airway, leading to the airflow obstruction characteristic of asthma . </P> <P> Goblet cell metaplasia in allergic asthma is due to the action of the cytokine IL - 13 . IL - 13 binds to the IL - 4Rα receptor and initates a STAT6 signalling response . Binding of IL - 13 causes phosphorylation of tyrosine residues at the IL - 4Rα . This results in docking of STAT6 monomers, which themselves are phosphorylated and then subsequently leave the receptor and congregate form STAT6 homodimers in the cytoplasm . These homodimers then enter the nucleus, where they bind to regulatory elements in the DNA, which affects the transcription of certain genes involved in mucus production . </P> <P> Induction of STAT6 signaling by IL - 13 leads to increased of expression of 15 - lipoxygenase (15 - LO - 1), which is an enzyme involved in the breakdown of unsaturated fatty acids . 15 - lipoxygenase acts by binding to phospholipids and yields hydroperoxy and epoxy metabolites . One such metabolite, 15 - hydroxyeicosatetranoic acid (15 - HETE), is released intracellularly, where it conjugates to phosphatidylethanolamine, a phospholipid component . 15 - HETE - PE induces expression of the mucin MUC5AC . </P>

Where do goblet cells occur in the respiratory system and what function do they serve