Leukocytes

Your immune system consists of cells that are made in your bone marrow and travel through your blood to the place where they are needed. Together they are called white blood cells or leukocytes, and there are five types, each with its own task. Neutrophils are the largest group and attack bacteria, lymphocytes recognise viruses and remember them, monocytes clean up, eosinophils respond to allergy and parasites, and basophils are the smallest group and release histamine. The total count is the sum of those five. But only a small part of your white cells is in your blood at any moment. The rest lies ready in your bone marrow and along the wall of your blood vessels, and can be pushed into the bloodstream within minutes. That happens with stress, with hard exercise and with an infection: adrenaline and cortisol loosen the cells from the vessel wall and the bone marrow releases extra cells. That is why the count can double within an hour without more cells having been made, and falls just as fast again. Your report shows the count in ×10⁹/L, billions of cells per litre. Normal is about 4.3 to 10.0 ×10⁹/L, where the labs overlap. A report with only the total says little; the differential, the count per type, says whether it is about bacteria, a virus, an allergy or something else.

The total of white blood cells is the first question in someone who is ill: is there an infection, and if so, which way is it going? A bacterium lifts the count through the neutrophils and usually goes with a high CRP; a virus often lets the total fall instead, while the lymphocytes rise. Those two patterns are so different that a blood count with differential often says more than the total. For anyone who is healthy, the count is interesting for another reason. A total that sits at the high end year after year, without an infection, is a sign of a body in a light inflammatory state. Smoking is the most common reason for that: smokers have persistently more white cells in their blood, and that corrects over months after stopping. Belly fat does the same in milder form, and poor sleep and long-term stress lift the count through cortisol. A higher total within the band therefore goes with more cardiovascular disease, not because the cells do harm, but because they show that the inflammation is there. A low total weighs more heavily than a high one. Below 4.0 ×10⁹/L you have fewer immune cells available, and if that is down to the neutrophils, you are more susceptible to bacteria. Usually the reason is harmless and temporary, a virus that has just passed or a medicine, and in some people a lower count is simply their make-up. If it stays low without explanation, or keeps falling, then a look at the bone marrow belongs with it.

A low total usually means the neutrophils have fallen, because they are the largest group. Look at the differential to see that. The most common reasons: a viral infection in the past weeks, which slows the bone marrow for a few days and keeps the count low for up to a few weeks, and medicines such as some painkillers, antibiotics, thyroid blockers and drugs for rheumatoid arthritis or epilepsy. In people with African or Middle Eastern ancestry the neutrophil count is naturally lower, without the immune system working any less well. A total that repeatedly stays below 4.0 ×10⁹/L without explanation, or together with anaemia and few platelets, calls for a look at the bone marrow. A high total means your body is mobilising cells, and the question is what for. With fever and a high CRP it is a bacterial infection or an inflammation. Without fever, stress, a hard training session the day before, a night of poor sleep, smoking, prednisone or a pregnancy are the most common explanations, and you see those in the differential as a rise in neutrophils without young forms. A total above 30.0 ×10⁹/L, or a rise in which the lab sees immature cells, is a different picture and is assessed the same day. Read the total beside the differential, because the five types each point a different way, and beside CRP, which says whether an infection is the cause. Platelets and haemoglobin from the same blood count show whether the bone marrow as a whole is working well.