The Differentiation of Vertebrate Immune Cells In the immune system, two types of cells participate directly in defense against pathogens. Plasma B cells produce and secrete immunoglobulins (antibodies), and killer T cell produce membrane-bound proteins that act as receptors for various substances. B cell antibodies and T cell receptors bind to specific antigens. A cell must make many varieties of these proteins because there are many potential pathogens.
An Antigen-Antibody Complex
Structure of an Antibody Molecule
Human Antibody Genes Two light chain loci: the on chromosome 2 and on chromosome 22 One heavy chain locus on chromosome 14. Each locus consists of a long array of gene segments.
Gene Segments for a Kappa Polypeptide An LV gene segment, encoding a leader peptide, which is removed later, and the N-terminal 95 amino acids of the variable region of the kappa light chain. (76 gene segments in humans; 40 of these are functional) A J gene segment, encoding the last 13 amino acids of the variable region of the kappa light chain. (5 gene segments in humans) A C gene segment, encoding the constant region of the kappa light chain. (1 gene segment in humans)
The Kappa Locus During B cell development, the kappa light chain gene that will be expressed is assembled from one LV segment, one J segment, and the C segment by somatic recombination. Segment joining is mediated by recombination signal sequences adjacent to each gene segment by a protein complex including RAG1 and RAG2 (recombination activating gene proteins 1 and 2).
Many Different Antibodies Can Be Produced 40 LV segments 5 J segments 1 C segment = 200 kappa light chains. Recombination of gene segments can create 120 lambda light chains and 6600 different heavy chains. Combinatorial assembly of these allows production of 2,112,000 different antibodies. Even more antibodies are possible due to variation in recombination sites and hypermutability of the variable regions.
Evidence for DNA Rearrangement During Immune Cell Differentiation