Thymus

The thymus is positioned in your chest, between your breast bone and your heart.  It is responsible for producing T-cells, and is especially important in newborn babies - without a thymus a baby's immune system collapses and the baby will die.  The thymus seems to be much less important in adults - for example, you can remove it and an adult will live because other parts of the immune system can handle the load.  However, the thymus is important, especially to T cell maturation.

Spleen

The spleen filters the blood looking for foreign cells (the spleen is also looking for old red blood cells in need of replacement).  A person missing their spleen gets sick much more often than someone with a spleen.

Bone marrow

Bone marrow produces new blood cells, both red and white. In the case of red blood cells the cells are fully formed in the marrow and then enter the bloodstream. In the case of some white blood cells, the cells mature elsewhere. The marrow produces all blood cells from stem cells. They are called "stem cells" because they can branch off and become many different types of cells - they are precursors to different cell types. Stem cells change into actual, specific types of white blood cells.

White blood cells

White blood cells are described in detail in the next section.