V(D)J recombination

V(D)J is the basis of the vertebrate immune system. It makes antibodies.
All vertebrates have an adaptive immune system. This protects them against many infections and diseases caused by viruses and bacteria. V(D)J is the basic mechanism. The work is done by developing lymphocytes (B cells and T cells). They make the immune system adaptive. What this means is that the body is able to defend against many infections it has never met before, and the defence gets better against repeat attacks from the same agent. 
The details are very complex. The system makes sequences of amino acids which bind on to the corresponding sequences of infective agents. The system also recognises "altered self cells" as seen in cancer.
Of course all systems have limits, so when the system fails to recognise a pathogen or cancer, we suffer the consequences.
In 1987 Susumu Tonegawa was awarded the Nobel Prize in Physiology or Medicine "for his discovery of the genetic principle for generation of antibody diversity".
The V(D)J refers to the way variable, joining and sometimes diversity regions are produced in the white blood cells. The cells are made in the bone marrow and thymus of vertebrates.