Why are opportunistic infections common in PLHIV?
In Study Session 20, you learned that CD4 lymphocytes (also known as CD4 cells) are a type of white blood cell with important functions in immunity. In this study session, we will use the simpler term, CD4 cells.
What is the function of CD4 cells in the immune system?
CD4 cells help to activate other white blood cells in the immune system, in the defence of the body against invasion by pathogens (Section 20.2.3).
Indeed, a reduced number of CD4 cells results in an impaired immune system. The lower the number of CD4 cells, the more impaired the immune system will be. Remember that HIV weakens the immune system precisely by infecting (and ultimately destroying) CD4 cells. Hence, the CD4 count (i.e. the number of CD4 cells in a specified volume of blood) is a good indicator of the 'health' of the immune system in PLHIV.
The CD4 counts of uninfected people usually fall between 800 and 1500 cells/mm³. At the early stages of the disease, HIV-infected people with immune systems that are functioning adequately have CD4 counts between 450 and 1500 cells/mm³.
The concentration of cells in the blood is usually expressed as the number of cells per cubic millimeter (cells/mm3), which is the same volume of blood as a millilitre (ml).
However, the risk of acquiring opportunistic infections increases proportionally to the decline in CD4 counts observed as the disease progresses. In other words, if the CD4 count falls below a certain limit, the immune system is unable to cope with invading pathogens and opportunistic infections become more frequent. In general, the following thresholds concerning CD4 counts are observed in adults and adolescents:
- When the CD4 count has decreased below 450 cells/mm³, a person living with HIV will start to acquire some mild or moderate opportunistic infections.
- When the CD4 count has decreased below 200 cells/mm³, a person living with HIV is highly likely to acquire severe opportunistic infections. It is at this stage that a person living with HIV is considered to have AIDS.
Based on your previous studies, why are CD4 counts determined in the blood, and not in other tissues and/or fluids such as urine?
CD4 cells reside primarily (although not exclusively) in the blood, where they are most likely to encounter invading pathogens (Section 20.2.2 and Figure 20.3).