What is a key characteristic of CD4 markers when used for immunophenotyping?

The key characteristic of CD4 markers in the context of immunophenotyping is that they are predominantly expressed on a subpopulation of T cells, specifically on T helper cells, rather than being universally expressed in all T cells. In a situation where CD4 is stained, T helper cells exhibit a "dim" expression pattern compared to other immune cells, which can be relevant in certain analytical techniques, such as flow cytometry, where distinguishing between subpopulations and their relative expression levels can provide valuable information about immune status.

CD4 markers play a vital role in the immune response by facilitating the activation of T cells and the regulation of immune responses. However, they do not label B cells nor are they absent in T cells. Instead, they highlight a specific characteristic of T helper cells, which is crucial for understanding their function within the immune system. Thus, the emphasis on their relative expression levels captures important nuances in immunophenotyping.