In flow cytometry, CD45 is often described simply as the “leukocyte marker.” However, its analytical value goes far beyond identifying white blood cells.
The intensity of CD45 expression, particularly when combined with forward scatter (FSC), side scatter (SSC), and lineage-specific markers, can provide valuable information for distinguishing leukocyte populations, evaluating maturation and identifying abnormal cell populations.
For this reason, CD45 remains a fundamental component of many flow cytometry immunophenotyping strategies.
CD45, also known as leukocyte common antigen (LCA) or PTPRC, is a transmembrane protein expressed on most hematopoietic cells.
Beyond its biological role in regulating immune-cell signaling, CD45 is highly useful analytically because its expression varies between different leukocyte populations and maturation stages.
This variation makes CD45 particularly valuable for population identification and gating.
One of the most common applications is the analysis of CD45 together with SSC.
While CD45 provides information about leukocyte identity and expression level, SSC provides information related to cellular complexity and granularity. Together, these parameters help create a map of the major leukocyte populations.
| Population | CD45 expression | SSC |
|---|---|---|
| Lymphocytes | Bright | Low |
| Monocytes | Intermediate | Intermediate |
| Granulocytes | Intermediate/bright | High |
| Immature or abnormal cells | Variable | Variable |
These patterns are useful as a starting point, but they should not be considered absolute. Sample type, maturation, disease state and technical conditions can influence CD45 expression.
A common mistake is to consider CD45 simply as positive or negative.
In reality, differences in CD45 intensity can help identify populations that deserve further investigation. For example, immature or abnormal hematopoietic cells may show reduced or heterogeneous CD45 expression compared with mature leukocytes.
This makes CD45 particularly useful in bone marrow analysis and hematological immunophenotyping.
CD45 is most powerful when combined with other markers.
For example, after identifying the leukocyte compartment, lineage-specific markers can be used to distinguish:
A panel combining CD45 + SSC + lineage-specific markers provides considerably more information than CD45 alone. This hierarchical approach also helps reduce the influence of debris and irrelevant events, improving the interpretation of complex samples.
A simplified workflow can be:
FSC/SSC → CD45-positive cells → CD45/SSC distribution → lineage-specific markers → detailed phenotyping
Depending on the assay, this can be complemented by singlet discrimination and viability markers.
The most important principle is that CD45 should be interpreted in context, rather than using a universal fluorescence threshold.
Some frequent errors include:
A robust analysis combines CD45 with other parameters and considers the biological characteristics of the sample.
CD45 is much more than a leukocyte identifier. Its expression intensity and relationship with SSC provide a valuable framework for identifying and characterizing different leukocyte populations. It can also help highlight immature or potentially abnormal populations that require further investigation.
For researchers and laboratory professionals, understanding CD45 gating is therefore an essential part of obtaining reliable and biologically meaningful results in flow cytometry immunophenotyping.