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CD Antigens: Clusters of Differentiation
A comprehensive guide to the surface proteins essential for identifying, classifying, and targeting immune cell populations in flow cytometry and pathology.
A Cluster of Differentiation (CD) antigen is a protein expressed on the surface of immune cells, used to classify and differentiate between different cell types based on their role in the immune response.
CD antigens serve as distinct molecular signatures that help researchers and clinicians identify specific stages of cell development, lineage, and activation states—particularly in leukocytes (white blood cells). Accurately tracking these markers is essential for diagnosing immune-related conditions, monitoring immunotherapies, and characterizing hematological malignancies.
1. Key Immune Cells & Their Primary CD Markers
The immune system relies on diverse populations of specialized cells. Below are the primary lineages and their defining CD signatures.
T Cells (T Lymphocytes)
All mature T cells express the CD3 complex. They are further divided by function:
- CD4: Found on Helper T cells (Th cells), which orchestrate the adaptive immune response.
- CD8: Present on Cytotoxic T cells (Tc cells), responsible for recognizing and destroying virus-infected or cancerous cells.
B Cells (B Lymphocytes)
Responsible for producing antibodies and maintaining immunological memory. CD19 and CD20 are the most common pan-B cell markers used from early development through maturity.
Myeloid & Innate Cells
- Natural Killer (NK) Cells: Essential innate effectors, typically defined by CD56 and CD16 expression (and lacking CD3).
- Monocytes & Macrophages: Key phagocytic cells classically identified by strong CD14 expression.
- Dendritic Cells: Professional antigen-presenting cells often expressing CD1c, CD80, and CD86.
2. Visualizing Lymphocyte Markers
Understanding the co-expression of these surface antigens is fundamental to accurate flow cytometry panel design and immunophenotyping.
3. Comprehensive CD Marker Dictionary
Below is a curated selection of commonly used and studied CD markers. Click any target to browse our highly validated recombinant proteins, primary antibodies, and fluorophore-conjugated clones.
| Antigen Name | Cellular Expression & Biological Function |
|---|---|
| CD1 | Found on dendritic cells and some T cells; involved in presenting lipid antigens to T cells. |
| CD3 | A complex found on all T cells, integral to T cell receptor (TCR) signalling and activation. |
| CD4 | Expressed on helper T cells; critical for activating other immune cells and serves as the primary receptor for HIV. |
| CD5 | Found on T cells and a small subset of B cells (B1 cells); involved in signalling and immune response regulation. |
| CD8 | Present on cytotoxic T cells; responsible for recognizing MHC Class I molecules and killing virus-infected or cancerous cells. |
| CD10 | Expressed on early B and T cells, as well as certain lymphomas (e.g., follicular lymphoma); widely used in cancer diagnosis. |
| CD11a | Expressed on leukocytes; pairs with CD18 to form LFA-1, heavily involved in cell adhesion and migration. |
| CD14 | Found on monocytes and macrophages; recognizes bacterial LPS components and plays a critical role in innate immune responses. |
| CD16 | Present on natural killer (NK) cells and some macrophages; an Fc receptor involved in antibody-dependent cellular cytotoxicity (ADCC). |
| CD19 | A key pan-B cell marker; used extensively in diagnosing B-cell malignancies and as a target for CAR-T cell therapies. |
| CD20 | Expressed on mature B cells; a primary clinical target in therapies for B-cell lymphomas and autoimmune diseases (e.g., Rituximab). |
| CD21 | Found on mature B cells; acts as a receptor for the Epstein-Barr virus (EBV) and the complement component C3d. |
| CD25 | The IL-2 receptor alpha chain. A classic marker for regulatory T cells (Tregs) and activated T cells; important in immune tolerance. |
| CD28 | Present on T cells; provides the critical costimulatory "Signal 2" required for full T cell activation and survival. |
| CD34 | Found on hematopoietic stem cells (HSCs) and progenitor cells; heavily used to identify and isolate stem cells for bone marrow transplants. |
| CD40 | Expressed on B cells, macrophages, and dendritic cells; binding to CD40L (CD154) is essential for activating B cells and class switching. |
| CD45 | The Leukocyte Common Antigen (LCA). Found on all nucleated hematopoietic cells; plays a critical role in antigen receptor signalling. |
| CD56 | NCAM (Neural Cell Adhesion Molecule). The archetypal marker for natural killer (NK) cells and certain subsets of NKT cells. |
| CD69 | An early activation marker rapidly upregulated on T cells, B cells, and NK cells following stimulation. |
| CD79a | Expressed strictly on B cells; forms a heterodimer with CD79b to mediate essential B cell receptor (BCR) intracellular signalling. |
| CD80 | Present on professional antigen-presenting cells (APCs); binds to CD28 to provide necessary costimulatory signals to T cells. |
| CD86 | Also found on APCs; works synergistically with CD80 to stimulate T cell activation. |
| CD95 (Fas) | The Fas receptor. Found on a variety of cells; engagement by FasL induces the extrinsic apoptosis (programmed cell death) pathway. |
| CD117 (c-Kit) | A marker for hematopoietic stem cells, mast cells, and certain cancers (e.g., GISTs); involved in cell survival and proliferation. |
| CD138 (Syndecan-1) | Expressed heavily on plasma cells (terminally differentiated B cells); universally used to identify and diagnose multiple myeloma. |