Two different types of blood cells (or haemozytes) circulate in the fluid: amoebocytes and nephrocytes.

When these cells circulate in an area that is slightly irritated they might provoke a reaction.

Long after a woman has given birth, cells from her fetus may still circulate in her blood stream, and a new study being reported today has linked those cells to development of a disease in the mother years after pregnancy.

Studies show that moderate exercise, however, helps immune cells circulate through the bloodstream at a more rapid pace, making it less likely that bacteria or viruses will slip through unnoticed.

Since the 1970's, researchers have been trying to establish whether fetal cells always circulate in the maternal blood and, if so, whether there are enough for analysis.

But hemoglobin's hold on the oxygen is just weak enough that needy cells in the body can grab much of it away as the red cells circulate through the bloodstream.

The cells develop in the bone marrow and circulate for about 100-120 days in the body before their components are recycled by macrophages.