Chapter 4 - Antibodies: Structure And Function

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Plasma

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the cell-free, fluid portion of blood, which contains all the clotting factors

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Serum

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fluid portion of the blood, which is free of all cell and clotting factors

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Gamma-gobulin fraction

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the electrophoretic fraction of serum that contains most of the immunoglobulin classes

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Immunoglobulin

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an antibody or a heavy or light polypeptide chain that is part of an antibody molecule

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Light (L) chains

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immunoglobulin polypeptides of the lambda or kappa type that join with heavychain polypeptides to form the antibody heterodimer

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Heavy (H) chains

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the larger polypeptide of an antibody molecule; it is composed of one variable domain and three or four constant domains;
there are five major classes of heavy chains in humans which determines the isotype of an antibody

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Fab fragment

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"fragment antigen binding";
a monovalent antigen binding fragment of an immunoglobulin molecule that consists of one light chain and part of one heavy chain, linked by an interchain disulfide bond

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Fc fragment

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"fragment crystallizable"; a crystallizable, non-antigen-binding fragment of an immunoglobulin molecule that consists of the carboxyl terminal portions of both heavy chains and possess binding sites for Fc receptors and the C1q component of complement

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Multiple myeloma

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a plasma cell cancer

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Myeloma cell

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a cancerous plasma cell

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Myeloma protein

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a monoclonal immunoglobulin produced by a myeloma cell

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Benze-Jones proteins

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protein found in high concentrations in the urine of patients with multiple myeloma; usually an Ig light chain or fragment thereof

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Plasmacytomas

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a plasma-cell cancer

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Variable (V) regions

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the amino terminal domain of an immunoglobulin molecule;
it consists of 100-110 amino acids and contains the CDRs that make up the antigen binding site

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Constant (C) regions

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the nearly invariant portion of the immunoglobulin molecule that does not contain antigen-binding domains;
the sequence of amino acids in the constant region determines the isotype of heavy chains (alpha, gamma, delta, epsilon, mu)and the type of light chains (lambda, kappa)

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Isotype

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an antibody class which is determined by the constant region sequence of the heavy chain;
the five human isotypes exhibit both structural and functional differences;
also refers to the set of isotypic determinants that is carried by all members of a species

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Immunoglobulin fold

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characteristic structure in immunoglobulins that consists of a domain of 100-110 amino acids folded into two beta pleated sheets, each containing three or four antiparallel beta strands, and stabilized by an intrachain disulfide bond

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Hypervariable regions

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one of three regions within the variable domain of each chain in immunoglobulins and T-cell receptors that exhibits the most sequence variability and contributes the most to the antigen-binding site;
also called CDR

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Complementarity-determining regions (CDRs)

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portions of the variable regions of antibody moleculesthat protrude from the V domains and have the potential to contact antigens;
the antigen-binding sites of antibody molecules are composed of CDRs

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Framework regions (FR)

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a relatively conserved sequence of amino acids located on either side of the hypervariable regions in the variable domains of immunoglobulin heavy and light chains

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Hinge regions

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the segment of an immunoglobulin heavy chain between the Fc and Fab regions;
it gives flexibility to the molecule and allows the two antigen-binding sites to function independently

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Secreted immunoglobulins (sIg)

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the form of antibody that is secreted by cells of the B lineage, especially plasma cells;
this form of Ig lacks a transmembrane domain

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Membrane bound immunoglobulins (mIg)

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a form of an antibody that is bound to a cell as a transmembrane protein;
acts as the antigen-specific receptor of B cells

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Opsonization

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deposition of opsonins on an antigen, thereby promoting a stable adhesive contact with an appropriate phagocytic cell

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Complement system

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a group of serum proteins that participates in an enzymatic cascade, ultimately generating the cytolytic MAC

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Antibody-dependent cell-mediated cytotoxicity (ADCC)

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a cell mediated reaction in which nonspecific cytotoxic cells that express Fc receptors (e.g., NK cells, neutrophils, macrophages) recognize bound antibody on a target cell and subsequently cause lysis of the target cell

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Transcytosis

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the movement of antibody molecules (polymeric IgA or IgM) across epithelial layers mediated by the poly-Ig receptor

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Passive immunization

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the acquisition of immunity by the receipt of preformed antibodies rather than by active production of antibodies after exposure to antigen

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J (joining) chain

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a polypeptide that links the heavy chains of monomeric units of polymeric IgM and di- or trimeric IgA;
the linkage is by disulfide bonds between the J chain and the carboxyl terminal cysteines of IgM or IgA heavy chains

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Poly-Ig receptor

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a receptor for polymeric Ig molecules that is expressed on the basolateral surface of most mucosal epithelial cells;
it transports polymeric Ig across epithelia

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Polyclonal antibody

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a mixture of antibodies produced by a variety of B-cell clones that have recognized the same antigens;
although all of the antibodies react with the immunizing antigen, they differ from each other in amino acid sequence

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P-K reaction

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local skin reaction to allergen by a normal subject at the site of injected IgE from an allergic individual;
no longer used

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Allotype

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a set of allotypic determinants characteristic of some but not all members of a species

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Idiotope

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a single antigenic determinant in the variable domains of an antibody or T-cell receptor;
generated by the unique amino acid sequence specific for each antigen

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Idiotype

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the set of antigenic determinants (idiotopes) characterizing a unique antibody or T-cell receptor

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B-cell receptor (BCR)

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complex comprising a membrane-bound immunoglobulin molecule and two associated signal-transudcing Igalpha/Igbeta molecules

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Fc receptor (FcR)

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cell-surface receptor specific for the Fc portion of certain classes of immunoglobulin;
present on lymphocytes, mast cells, macrophages, and other accessory cells

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Neonatal Fc receptor (FcRn)

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an MHC class I-like molecule that controls IgG half-life and transports IgG across the placenta

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Immunoglobulin superfamily

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group of proteins that contain immunoglobulinfold domains,or structurally related domains;
includes immunoglobulins, T-cell receptors, MHC molecules, and numerous other membrane molecules

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Monoclonal antibody

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homogeneous preparations of antibody molecules, produced by a single clone of B lineage cells, often a hybridoma, all of which have the same antigenic specificity

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Hybridoma

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a clone of hybrid cells formed by fusion of normal lymphocytes with myeloma cells;
it retains the properties of the normal cell to produce antibodies or T-cell receptors but exhibits the immortal growth characteristic of myeloma cells;
used to produce monoclonal antibodies

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Immunotoxins

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highly cytotoxic agent produced by conjugating an antibody with highly toxic agent, usually a protein such as ricin;
contains two peptides, an inhibitory chain and a membrane-binding chain

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Abzyme

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a monoclonal antibody that has catalytic activity

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Allotypic determinant

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an antigenic determinant that varies among members of a species;
the constant regions of antibodies possess allotypic determinants