Much of our immune defense against invading organisms is predicated upon the tremendous diversity of immunoglobulin molecules. Immunoglobulins are glycoproteins produced by B lymphocytes and plasma cells. These molecules can be considered receptors because the primary function of the immunoglobulin molecule is to bind antigen. A single person can synthesize 10 to 100 million different immunoglobulin molecules, each having a distinct antigen-binding specificity. The great diversity in this so-called humoral immune system allows us to generate antibodies specific for a variety of substances, including synthetic molecules not naturally present in our environment. Despite the diversity in the specificities of antibody molecules, the binding of antibody to antigen initiates a limited series of biologically important effector functions, such as complement activation and/or adherence of the immune complex to receptors on leukocytes. The eventual outcome is the clearance and degradation of the foreign substance. This chapter describes the structure of immunoglobulins and outlines the mechanisms by which B cells produce molecules of such tremendous diversity with defined effector functions.
Acronyms and Abbreviations
ADCC, antibody-dependent cellular cytotoxicity; AID, activation-induced deaminase; BACH2, basic leucine zipper transcription factor 2; BCL-6, B-cell chronic lymphocytic leukemia/lymphoma 6; BiP, immunoglobulin-binding protein; Blimp-1, B-lymphocyte-induced maturation protein-1; BLNK, B-cell linker protein; BTK, Bruton tyrosine kinase; C, constant; CDR, complementarity determining region; CRI, cross-reactive idiotype; CSR, class switch recombination; D, diversity; DLBCL, diffuse large B-cell lymphoma; DNA-PK, DNA protein kinase; E2F1, E2F transcription factor 1; EBF1, early B-cell factor 1; ERGIC, ER-Golgi-intermediate compartment; FR, framework region; H, heavy; HMG, high-mobility group protein; Ig, immunoglobulin; IL, interleukin; IRF4, interferon regulatory factor 4; ITAM, immunoreceptor tyrosine-based activation motif; κ, immunoglobulin kappa light chain; Kde, kappa-deleting element; λ, immunoglobulin lambda light chain; L, light; MITF, microphthalmia-associated transcription factor; MYBL1 and 2, v-myb myeloblastosis viral oncogene homologues 1 and 2; NHEJ, nonhomologous DNA end-joining; PAX5, paired box gene 5; PDI, protein disulphide isomerase; PLC, phospholipase C; POU2AF1, Pou domain, class 2, associating factor 1; POU2F2, Pou domain, class 2, factor 2; PRDM1, positive regulatory domain 1-binding factor-1; RAG, recombination-activating gene; RSS, recombination signal sequence; SCID, severe combined immunodeficiency; SHP-1, Src homology 2 domain-containing protein tyrosine phosphatase-1; SHIP-1, phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase 1; TCFE2A, transcription factor E2a; UNG, uracil-DNA glycosylase; V, variable-region gene; V(D)J, exon created by a rearranged immunoglobulin heavy-chain variable-region gene, diversity gene segment, and joining gene segment; XBP1, X-box binding protein-1.
IMMUNOGLOBULIN STRUCTURE AND FUNCTION
All naturally occurring immunoglobulin molecules are composed of one or several basic units consisting of two identical heavy (H) chains and two identical light (L) chains (Fig. 75–1).1 The four polypeptides are held in a symmetrical, Y-shaped structure by disulfide bonds and noncovalent interactions.2,3,4 The internal disulfide bonds of the heavy and light chains cause the polypeptides to fold into compact globe-shaped regions called domains, each containing approximately 110 to 120 ...