Biological role and dynamics of expression
CD antigens are not simply passive, static "markers." They are active participants in immunological processes that perform multiple tasks. Primarily, they act as receptors, ligands, and adhesion molecules, ensuring full intercellular interaction within the immune system.
The set of cluster of differentiation molecules on the cell surface is a dynamic indicator. Their expression directly depends on several factors:
- The differentiation stages of a specific cell.
- The degree of cellular maturity.
- Belonging to a specific subpopulation.
These markers are present on virtually all immune cells: T- and B-lymphocytes, macrophages, dendritic cells, and neutrophils. In clinical practice, the use of monoclonal antibodies to CD antigens allows physicians and researchers to accurately distinguish cell populations and assess the degree of cell differentiation in detail.
World Health Organization nomenclature
The classification of CD antigens approved by the WHO is based on the specificity of antibodies specifically to human leukocyte antigens.
The principle of nomenclature formation is extremely simple: each newly discovered molecule is assigned its own sequential number (for example, CD1, CD2, and so on). To date, the list is remarkably extensive, including hundreds of entries and exceeding 300 markers.
Interestingly, although initially created for white blood cells, this classification has proven its universality. It now encompasses not only leukocytes but also specific surface antigens of many other cells in the body.
Characteristics of T- and B-lymphocyte markers
Each lymphocyte subpopulation possesses a strictly defined set of molecules that determines its functions.
T-lymphocyte markers:
- CD3: a general (so-called pan-T) marker of absolutely all T-lymphocytes. It is part of the T-cell receptor (TCR) and is responsible for transmitting an activating signal inside the cell upon antigen binding.
- CD4: the classical marker of T-helper cells. It is a costimulatory molecule whose main function is to recognize MHC class II molecules on the surface of antigen-presenting cells. An important nuance: the CD4 marker is also expressed on monocytes, macrophages, dendritic cells, and even certain tumor cells.
- CD8: the marker of cytotoxic T-lymphocytes (CTLs). It is also a costimulatory molecule, but unlike CD4, its function is to recognize MHC class I molecules on target cells.
- CD28: a critical costimulatory molecule of T-lymphocytes, without which their full activation is impossible.
B-lymphocyte markers: The main specific markers of B cells are CD19, CD20, and CD21. Their primary task is direct participation in the complex processes of activation and subsequent proliferation of B-lymphocytes.
Markers of NK cells, activation, stem cells, and apoptosis
In addition to standard T- and B-cell markers, there are specialized antigens responsible for specific functions or states:
- CD16 (natural killer cell marker): a low-affinity receptor for immunoglobulin G (IgG). Its presence enables NK cells to participate in antibody-dependent cell-mediated cytotoxicity (ADCC).
- CD25 (activation marker): a molecule that appears on the surface of both T- and B-lymphocytes upon their activation.
- CD34: a key marker used to identify hematopoietic stem cells (HSCs).
- CD40: a costimulatory molecule necessary for productive interaction between antigen-presenting cells (APCs) and B-lymphocytes.
- CD95 (Fas/APO): a specific marker acting as a receptor for the Fas ligand. It functions as a "Death Receptor" (DR). Its structural feature is the presence of a specialized intracellular "Death Domain" (DD). Upon activation, this specific domain triggers the mechanism of apoptosis—programmed cell death.
Briefly: answers to topical questions
Which CD markers are specific to B-lymphocytes?
Specific markers of B-lymphocytes are molecules involved in their activation, proliferation, and antigen recognition. The main B-cell antigens include:
- CD19 — a strictly specific marker for all B-lymphocytes.
- CD20 — a specific marker involved in activation processes.
- CD21 (CR2) — a complement receptor.
- CD22 — a marker with pronounced expression on B cells.
- CD23 — a marker of follicular B2 cells.
- CD79a and CD79b (Igα and Igβ) — polypeptide chains within the BCR complex.
CD32, CD43, CD45, CD62L, CD72, and CD81 may also be expressed on B cells.
Which CD markers belong to the immunoglobulin superfamily?
The immunoglobulin superfamily includes various receptors, differentiation markers, and adhesion molecules. This family encompasses the following CD markers:
- CD2, CD3, CD4, CD8 — T-cell coreceptors and differentiation markers.
- CD28, CD152 (CTLA-4), ICOS, PD-1 — costimulatory or regulatory molecules.
- CD54 (ICAM-1) and CD106 (VCAM-1) — adhesion molecules.
- CD79a (Igα) and CD79b (Igβ) — polypeptide chains of B-cell receptor heterodimers.
- CD90 (Thy-1) — a member of the immunoglobulin superfamily.
- CD121 (IL-1R) — a cytokine receptor.
Which CD markers function as adhesion molecules?
Adhesion functions are performed by integrins, selectins, and members of the immunoglobulin superfamily that mediate migration and intercellular contacts. CD markers associated with adhesion include:
- CD11a/CD18 (LFA-1), CD11b/CD18 (Mac-1, CR3), CD11c/CD18 (p150/p95, CR4) — leukocyte $eta_2$-integrins.
- CD18 — the common $eta_2$-chain of integrins.
- CD54 (ICAM-1) and CD106 (VCAM-1) — immunoglobulin-like adhesion molecules.
- CD62L (L-selectin) — an adhesion molecule on naive T cells involved in migration.
- CD15s — an L-selectin receptor, the altered expression of which is associated with leukocyte rolling defects.
- CD34 — an endothelial molecule that interacts with L-selectin during neutrophil capture.
Which CD markers are expressed on the surface of natural killer (NK) cells?
The following markers are expressed on the surface of natural killer (NK) cells:
- CD16 — a low-affinity receptor for IgG involved in antibody-dependent cell-mediated cytotoxicity.
- CD56 and CD94 are characteristic surface markers of NK cells.
- CD2 is a marker expressed by NK cells.
- CD8 is expressed on a subset of natural killer cells.
- CD21 (CR2) is a complement receptor that can be expressed on natural killer cells.
At the same time, NK cells lack T-cell receptor (TCR) markers.
Which molecules are required for the co-stimulation of T lymphocytes upon their activation?
For T-lymphocyte activation, interactions between co-stimulatory molecules on T cells and their corresponding ligands on antigen-presenting cells (APCs) are essential. The main co-stimulatory pairs include:
- CD28 on the T cell interacts with CD80 (B7-1) and CD86 (B7-2) on the APC; this provides co-stimulation and IL-2 secretion.
- CD154 (CD40L) on the T cell binds to CD40 on the APC or B cell.
- ICOS on the T helper cell interacts with ICOS-L on the APC.
- OX40 on the T helper cell interacts with OX40L on the APC.
Adhesion molecules are also involved in T-B cell cooperation.
Which CD markers appear on lymphocytes at the moment of their activation?
During the activation of T lymphocytes, specific markers appear sequentially on the membrane:
- CD69 is the earliest activation antigen, appearing on the surface within 2–3 hours.
- CD25 is an early activation marker, the IL-2 receptor alpha chain, appearing after 4 hours. CD25 is also listed as an activation marker appearing on T and B lymphocytes.
- CD71 is the transferrin receptor; it is expressed in approximately 24 hours and is essential for proliferation.
- MHC-II molecules are late T-cell activation markers; they are expressed on days 3–6.
- CD95 can be detected on activated T cells; CD95 is the Fas/APO receptor, a "death receptor" that triggers apoptosis.
Adhesion molecules, including VLA group beta1-integrins, also appear during the late stages of activation.
B-lymphocyte markers are easy to remember because they are three consecutive numbers: CD19, CD20, CD21.
Class and exam questions
Can the T-helper marker (CD4) be detected on non-lymphoid cells?
Yes, in addition to T-helper cells, the CD4 molecule is also expressed on the surface of monocytes, macrophages, dendritic cells, and in some cases, tumor cells.
What is the function of the CD3 molecule?
CD3 is a common pan-T marker for all T lymphocytes. As part of the T-cell receptor (TCR) complex, this molecule transmits an activating signal into the cell upon successful antigen binding.
What happens when the CD95 receptor interacts with its ligand?
CD95 (Fas) functionally acts as a "death receptor." Upon binding to the Fas ligand, its intracellular "death domain" (DD) is activated, leading to the initiation of apoptosis.
What else to study on this topic
- Detailed structure of the T-cell receptor (TCR) and the role of the CD3 molecule in signal transduction
- The mechanism of recognition of MHC class I and II molecules with the participation of CD8 and CD4 co-receptors
- The process of antibody-dependent cell-mediated cytotoxicity (ADCC) involving CD16
- The interaction between CD40 and its ligand on antigen-presenting cells and B lymphocytes
- The intracellular apoptotic cascade triggered by the "death domain" (DD) of the CD95 receptor
Study the full topic in the «Microbiology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Clusters of Differentiation" and 4,383 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 141 video lectures on the subject
Other topics in "Immunology"
Free video lessons: microbiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.