Structure of FcεR1 on<unk> . FcεR1 is a tetramer made of one alpha (α) chain, one beta (β) chain, and two gamma (γ) chains. IgE is binding to α chain, signal is transduced by ITAM motifs on β and γ chains. Structure of the high-affinity IgE receptor, FcεR1 FcεR1 is a high affinity IgE-receptor that is expressed on the surface of the<unk> . FcεR1 is a tetramer made of one alpha (α) chain, one beta (β) chain, and two identical, disulfide-linked gamma (γ) chains. The binding site for the IgE is formed by the extracellular portion of the α chain that contains two domains that are similar to Ig. One transmembrane domain contains an aspartic acid residue, and one contains a short cytoplasmic tail. The β chain contains, a single immunoreceptor tyrosine-based activation motif ITAM, in the cytoplasmic region. Each γ chain has one ITAM on the cytoplasmic region. The signaling cascade from the receptor is initiated when the ITAMs of the β and γ chains are phosphorylated by tyrosine. This signal is required for the activation of<unk> s. Type 2 helper T cells,(Th2) and many other cell types lack the β chain, so signaling is mediated only by the γ chain. This is due to the α chain containing endoplasmic reticulum retention signals that causes the α-chains to remain degraded in the ER. The assembly of the α chain with the co-transfected β and γ chains mask the ER retention and allows the α β γ complex to be exported to the golgi apparatus to the plasma membrane in rats. In humans, only the γ complex is needed to counterbalance the α chain ER retention. Allergen process Allergen-mediated FcεR1 cross-linking signals are very similar to the signaling event resulting in antigen binding to lymphocytes. The Lyn tyrosine kinase is associated with the cytoplasmic end of the FcεR1 β chain. The antigen cross-links the FcεR1 molecules, and Lyn tyrosine kinase phosphorylates the ITAMs in the FcεR1 β and γ chain in the cytoplasm. Upon the phosphorylation, the Syk tyrosine kinase gets recruited to the ITAMs located on the γ chains. This causes activation of the Syk tyrosine kinase, causing it to phosphorylate. Syk functions as a signal amplifying kinase activity due to the fact that it targets multiple proteins and causes their activation. This antigen stimulated phosphorylation causes the activation of other proteins in the FcεR1-mediated signaling cascade. Degranulation and fusion An important adaptor protein activated by the Syk phosphorylation step is the linker for activation of T cells (LAT). LAT can be modified by phosphorylation to create novel binding sites. Phospholipase C gamma (PLCγ) becomes phosphorylated once bound to LAT, and is then used to catalyze phosphatidylinositol bisphosphate breakdown to yield inositol trisphosphate (IP3) and diacyglycerol (DAG). IP3 elevates calcium levels, and DAG activates protein kinase C (PKC). This is not the only way that PKC is made. The tyrosine kinase FYN phosphorylates Grb2-associated-binding protein 2 (Gab2), which binds to phosphoinositide 3-kinase, which activates PKC. PKC leads to the activation of myosin light-chain phosphorylation granule movements, which disassembles the actin–myosin complexes to allow granules to come into contact with the plasma membrane. The<unk>  granule can now fuse with the plasma membrane. Soluble N-ethylmaleimide sensitive fusion attachment protein receptor SNARE complex mediates this process. Different SNARE proteins interact to form different complexes that catalyze fusion. Rab3 guanosine triphosphatases and Rab-associated kinases and phosphatases regulate granule membrane fusion in resting<unk> s. MRGPRX2<unk>  receptor Human<unk> -specific G-protein-coupled receptor MRGPRX2 plays a key role in the recognition of pathogen associated molecular patterns (PAMPs) and initiating an antibacterial response. MRGPRX2 is able to bind to competence stimulating peptide (CSP) 1 - a quorum sensing molecule (QSM) produced by Gram-positive bacteria. This leads to signal transduction to a G protein and activation of the<unk> .<unk>  activation induces the release of antibacterial mediators including ROS, TNF-α and PRGD2 which institute the recruitment of other immune cells to inhibit bacterial growth and biofilm formation. The MRGPRX2 receptor is a possible therapeutic target and can be pharmacologically activated using the agonist compound 48/80 to control bacterial infection. It is also hypothesised that other QSMs and even Gram-negative bacterial signals can activate this receptor. This might particularly be the case during Bartonella chronic infections where it appears clearly in human symptomatology that these patients all have a<unk>  activation syndrome due to the presence of a not yet defined quorum sensing molecule (basal histamine itself?). Those patients are prone to food intolerance driven by another less specific path than the IgE receptor path: certainly the MRGPRX2 route. These patients also show cyclical skin pathergy and dermographism, every time the bacteria exits its hidden intracellular location. Enzymes Enzyme Function Lyn tyrosine kinase Phosphorylates the ITAMs in the FcεR1 β and γ chain in the cytoplasm. It causes Syk tyrosine kinase to get recruited to the ITAMS located on the γ chains. This causes activation of the Syk tyrosine kinase, causing it to phosphorylate Syk tyrosine kinase Targets multiple proteins and causes their activation Phospholipase C Catalyzes phosphatidylinositol 4,5-bisphosphate Inositol trisphosphate Elevates calcium levels Diacylglycerol Activates protein kinase C FYN Phosphorylates GAB2 GAB2 Binds to phosphoinositide 3-kinase Phosphoinositide 3-kinase Activates protein kinase C Protein kinase C Activates myosin light-chain phosphorylation granule movements that disassemble the actin-myosin complexes Rab-associated kinases and phosphatases Regulate cell granule membrane fusion in resting<unk> s
Mast cell
T cell
Reactive lymphocyte
Dendritic cell
Natural killer cell
Prolymphocyte
Nonspecific immune cell
Basophil