Monocytes

After the first wave of neutrophils comes the second: monocytes. They are the largest white blood cells, and in your blood they are in transit. After about three days they crawl through the vessel wall into the tissue and change there into macrophages, literally big eaters, which digest dead cells, bacteria and debris and release signal substances that set repair in motion. Every organ has its own macrophages: in the lungs, the liver, the brain and the vessel wall, and they are replenished from the blood. Because monocytes are only needed after the first wave, the count in your blood rises later than the neutrophils and the CRP. It rises in the recovery phase of an infection, when clearing up is needed, and it stays high as long as an inflammation continues: with a bowel inflammation, rheumatoid arthritis, an infection that persists such as tuberculosis or an infected heart valve. Prednisone and chemotherapy suppress the count. Your report shows the count in ×10⁹/L, and read it as an absolute count, not as a percentage. Normal is about 0.3 to 0.9 ×10⁹/L, where the labs overlap. The lower end says little: monocytes are scarce, and a low count is almost never a finding on its own.

Monocytes tell which phase an inflammation is in. With a fresh bacterial infection the neutrophils are high and the monocytes still normal; a week later, when you feel better, the neutrophils are back and the monocytes rise, because clearing up is under way. A count that stays high while CRP is already falling is therefore no reason for concern but the sign of recovery. A count that stays high together with CRP points to an inflammation that does not stop, and then the question is where it sits. For anyone who is healthy, monocytes are interesting because they are the cell that builds atherosclerosis. Macrophages in the vessel wall swallow LDL particles that have lodged there, fill up with fat and become foam cells, the core of a plaque. More monocytes in your blood means more supply for that process, and a higher count within the band therefore goes with more cardiovascular disease. Smoking and belly fat lift the count persistently, exactly the two things that also strain the vessel wall, and after stopping smoking and losing belly fat the count falls over months. The rare side is a count that stays far above the band without an infection or inflammation. That can be a bone marrow disease, in which the production of monocytes goes astray, and then you usually see abnormalities in the other cells too. That is the reason a persistently high count is checked, even when you feel well.

A low count almost never means anything on its own. The band starts low, and a count just below it comes from prednisone, chemotherapy or a severe acute infection in which the cells have been drawn en masse into the tissue. As soon as the cause is gone, the count comes back. Only a low count beside low neutrophils, anaemia and few platelets says something about the bone marrow as a whole. A high count means clearing up is under way, and the question is whether that is a phase or a state. Usually it is the first and rarely the last: the recovery phase after an infection, with CRP already falling; an inflammation that has been running a while, such as a bowel inflammation or rheumatoid arthritis; an infection that persists; smoking; and an autoimmune disease. A count that repeatedly stays far above the band without one of those explanations, and certainly with abnormalities in the other cells, should have the bone marrow looked at. Always read the absolute count: when the neutrophils fall, the percentage of monocytes rises without a single cell having been added. Monocytes belongs beside CRP, which says whether the inflammation is still active or already dying down, beside the neutrophils and lymphocytes, which show the phase of an infection, and beside the total of white cells.