"Topic 2: Basic concept of antigen receptor :B cells and T cells 1.Antigen recognition by B-cells and T -cells T Cell receptor:Structure:? The T cell receptors contains ? ? and ?? heterodimer domains inthem, due to this feature that they are similar to immunoglobulinsthey are classified into a group known as immunoglobulinsuperfamily.? T cell receptors consists oftwo domains that contains anintrachain disulfide bond that spans 60-75 amino acids and theyare (1) C domain or constant (2) V domain or variable? These domains resemble the Fab region of the immunoglobulin.? The variable regions of the t cell receptors have hypervariableregionswhich are similar to the complementary determiningregions.(CDR’S)?In addition to the above hypervariable regions there is onemore hypervariable regions in the ? chain that does not contactan antigen and hence not recognized as CDR.? Along with the constant domain, each TCR consists of ashortconnecting sequence ?, in which a cysteine residues forms adisulfide links with the other chain of the heterodimer.? In addition to this region, there is a transmembrane regionpresent containing 21 and 22 amino acids, which anchors eachchain in plasma membrane. ? The transmembrane region has an unique feature that is itcontains positively charged amino acid residues. ? These regions helps the T cell receptor heterodimer to interactwithchains of the signal transducing CD3 complex region .? Each TCR chain consists of a short cytoplasmic tail of 5-12amino acids at the carboxyl terminals.? Majority of RT cells that are expressed in humans are coded by? ? genes.? These receptors interact with peptide antigens and presented onthe surface for antogen presenting.? A cleft is present on the receptor which helps in accommodatingthe microbial phospholipids. Antigen recognition:? T cells recognize antigen fragments that have been bound to aself-cell protein called a major histocompatibility molecule.? MHC, major histocompatibility complex of genes codes for thesemolecules MHC molecules bind to antigen fragments inside aself-cell, and present the fragments at the surface of the cell.? T cells detect the presented antigen MHC comple ?x. B cell Receptor:Structure:? The B-cell receptor or BCR is a transmembrane receptor proteinlocated on theouter surface of the B cells.? The B-cells receptor includes CD79 and the immunoglobulin.? The B cell receptor extends both outside the cell and inside thecell (i.e, above and below the plasma membrane).? The B cell receptor has a unique antigen-binding site.? BCR is composed of two parts:1. A membrane bound immunoglobulinmolecule of one isotype.2. Signal transduction moiety -a heterodimer called Ig- ? / Ig- ?(CD79) bound together by disulfide bonds.3. Each member of the dimer spans the PM and has cytoplasmictails bearing an ITAM.Antigen recognition:? B cell receptors are Y-shaped.? Each branch of the “Y” has 2 parts, called chains Inner, heavychain makes the full “Y”Outer, light chain is located on thebranches of the “Y”? Both chains a are linked by chemical bonds.? The bottom of the “Y” is anchored in the B cell membrane.? The bottom regions of both chains have constant amino acidsequences.? The outer branches of both chains have variable amino acidsequences.? These variable ends are the antigen binding sites. ? They bind directly to the epitopes B cells recognize unalteredantigen ?s.2. Compare and contrast the development of B and Tcells.Property T cellsB cells OriginBone marrow in adultsBone marrow in adultsMaurationThymusLymphoid tissue or bonemarrow . Complement AbsentPresent receptors Surface AbsentPresent immunoglobul ins Type of Cell mediated and humoralHumoral immunity immunity Types of cellsMemory cells and plasma Tc cells,cellsTh cells,T regulator cells,Ts cells,TDTH cells,Null cells "