As soon as a foreign molecule enters the human body, the immune system immediately takes over.  The foreign body, also known as an antigen, can be anything from a harmless macromolecule to an infectious agent.  The body sends B-cells with surface receptor proteins, known as antibodies, to bind to the antigens with the hope of inducing a specific immune response (see diagram below).  Each B-cell has surface receptors that are slightly different from every other B-cell and specific to a select few number of antigens.  When an antigen binds to a B-cell, the cell activates and becomes a plasma cell capable of secreting a large number of antibodies.  These secreted antibodies strongly bind to the antigen, allowing the immune system to combat the foreign invader.

     Antibodies have a structure consisting of two identical heavy chains and two identical light chains of amino acids.  Because antibodies are proteins, they are encoded by genes.  By recombining the genes that encode for the heavy and light chains in different fashions, an enormous amount  of diversity is generated.

In germ line DNA, gene segments encode an antibody light or heavy chain, so that the antibody is then prepared to enter the germ cell.  Between the initial and final binding, further mutations occur on the germ line of the DNA that greatly increase the diversity and binding capabilities of the antibodies.