1. 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.

2. An Antigen-Antibody Complex

3. Structure of an Antibody Molecule

4. 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.

5. Gene Segments for a Kappa Polypeptide
An LV 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)

6. The Kappa Locus
During B cell development, the kappa light chain gene that will be expressed is assembled from one LV 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).

10. Many Different Antibodies Can Be Produced
40 LV 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.

11. Evidence for DNA Rearrangement During Immune Cell Differentiation