The thymus is an organ located in the upper side of the chest. Here is where the T cells are produced. The thymus is probably one of the most important parts of the immune system.
Another important part of the immune system is the B cell. It is not as important as the thymus. B cells are cells that mark the foreign particles with proteins called "antibodies". The antibodies cause other proteins in the blood to come in and destroy the foreign particles.
The Immune system is filled with cells. There is a group of cells that are all types of T cells. One type of T cell is a Killer T cell which defends the body by destroying body cells that have been infected with a foreign particle. Another type of T cell is the helper T cell which helps B cells fight off invaders. The last type of T cell is the T suppressor cell. T suppressor cells alert the body that the foreign particles have been destroyed and that everybody can relax. Those are the types of T cells.
Without a Immune system you would die once you touch a germ. There has been some cases in which there were babys that had no Immune system and they had to live in a bubble.