A CD4 count is the measured level of a certain type of protein, which is often present in certain types of white blood cells. CD4 stands for "cluster of differentiation four," and is an important component of the body's ability to produce an immune response to a would-be infection. A CD4 count can also refer to the specific lab work that is done to determine its level, as a routine part of treatment for someone with the humane immunodeficiency virus (HIV), which can lead to acquired immune deficiency syndrome (AIDS).
Whenever a person visits his HIV specialist, a CD4 count will be taken, to determine, at a glance, the condition of his immune system. CD4 is often present in T-cells and other cells which form and coordinate the body's immune system. The HIV virus can take over and use these cells to make copies of itself, which go on to use other immune cells to make other copies. For a while, the body can replace T-cells at a fast enough rate to combat this pattern. With proper treatment, this time period can be greatly extended, but the CD4 count is watched vigilantly as an indicator of the progression of HIV.
As HIV progresses, a person's CD4 count will go lower and lower, indicating that the presence of the virus is stronger. The results of the count are usually expressed in the number of cells per microliter of blood, an amount which is roughly the size of a pea. A person who is free of HIV will normally have upwards of 750 T-cells in this amount of blood. Someone who is HIV positive is considered to have a normal CD4 count if this number is around 500 in the same amount of blood. Should this number be lowered below 200, this will be an indication of serious immune damage.
A count below 200 is the threshold beyond which a person is medically defined as having AIDS, as opposed to just being infected with HIV. The CD4 count number, while important, also fluctuates considerably. For instance, a person's CD4 is usually higher in the evening than in the morning, and can be affected by the presence of an acute illness like the flu. Simple stress and fatigue can also affect the results. Thus, the CD4 count is useful over time to determine a pattern of immune system health, but the variability of this number over the course of even a single day should be taken into account when interpreting these test results.