STEPTRACKERIMMUNOLOGY + HEMATOLOGY | CELLS & ORGANS | HIGH-YIELD REVIEW

CD Markers & Surface Molecules

Cluster-of-differentiation numbers identify cells on flow cytometry and name the targets of many drugs — learn the markers by the cell that carries them.

FOCUS ON

All leukocytes: CD45. T cells: CD3 (TCR complex), CD4 (helper; binds MHC II), CD8 (cytotoxic; binds MHC I), CD28 (binds B7), CD40L (CD154; B-cell help), CD25 (IL-2Rα, Tregs). B cells: CD19, CD20, CD21 (C3d receptor, EBV receptor), CD40, MHC II, B7 (CD80/86). NK: CD16, CD56. Macrophages/monocytes: CD14 (LPS co-receptor), CD68. Stem cells: CD34. Platelets: GPIIb/IIIa (CD41/61). Inhibitory: CTLA-4, PD-1. Adhesion: integrins (LFA-1 = CD11a/CD18), selectins. Complement regulators: CD55 (DAF), CD59 → lost in PNH (GPI anchor).

MARKER

CELL

FUNCTION

CLINICAL LINK

CD3

All T cells

Signals for the TCR

T-cell lymphoma marker · muromonab (historic)

CD4

Helper T cells, monocytes, macrophages

Binds MHC II

HIV receptor (gp120)

CD8

Cytotoxic T cells

Binds MHC I

↑ in viral infection; atypical lymphocytes in mono

CD28 · CTLA-4 (CD152)

T cells

Bind B7 (CD80/86): CD28 activates, CTLA-4 inhibits

Abatacept (CTLA-4-Ig) blocks costimulation · ipilimumab blocks CTLA-4

PD-1 · PD-L1

Exhausted T cells · tumor cells, APCs

Inhibitory checkpoint

Nivolumab, pembrolizumab, atezolizumab

CD40L (CD154)

Activated CD4⁺ T cells

Binds CD40 on B cells → class switching

X-linked hyper-IgM syndrome

CD25

Tregs, activated T cells

IL-2 receptor α

Basiliximab · adult T-cell leukemia (HTLV-1) is CD25 ⁺

CD19 · CD20

B cells

BCR coreceptor · unknown signaling role

CAR-T (CD19) · rituximab, ocrelizumab (CD20)

CD21 (CR2)

B cells, follicular DCs

Binds C3d — boosts BCR signaling

EBV receptor

CD16 · CD56

NK cells (CD16 also on neutrophils)

CD16 = FcγRIII → ADCC

NK/T-cell lymphoma

CD14

Monocytes, macrophages

Co-receptor for LPS with TLR4

Septic shock signaling

CD34

Hematopoietic stem cells, endothelium

—

Stem cell harvest, AML marker

CD15 · CD30

Reed-Sternberg cells

—

Classic Hodgkin lymphoma · brentuximab (CD30)

CD18 (β2 integrin)

Leukocytes

With CD11 forms LFA-1, Mac-1 → firm adhesion

Leukocyte adhesion deficiency type 1

CD55 · CD59

All blood cells (GPI-anchored)

Block complement on self cells

PNH (PIGA mutation) → hemolysis, thrombosis

CD1a · CD117 · CD38

Langerhans cells · mast cells/GIST (KIT) · plasma cells

—

Daratumumab (CD38) in myeloma

Mechanism — signal 1 and signal 2

Signal 1TCR + CD3 binds peptide–MHC; CD4/CD8 stabilize

→

Signal 2CD28 binds B7 on APC

→

IL-2 → T-cell proliferation

BrakesCTLA-4 (outcompetes CD28 for B7), PD-1

→

Checkpoint inhibitors release brakesAnti-tumor + autoimmunity

Markers by lineage

LINEAGE

KEY MARKERS

DRUG TARGET

T cell

CD3, CD4/CD8, CD28, CD25

Basiliximab, abatacept

B cell

CD19, CD20, CD21, CD40

Rituximab, CAR-T

CD16, CD56

—

Myeloid

CD14, CD33, CD68

Gemtuzumab (CD33)

Plasma cell

CD38, CD138

Daratumumab

Stem cell

CD34

—

How it's tested

COMMON CONFUSIONCD21 (B-cell complement receptor, EBV entry) vs CD4 (HIV entry) — both viruses use an immune receptor to infect their target.

THE TRAPPNH red cells lack CD55/CD59 because the GPI anchor is missing — the marker proteins themselves are normal genes.

YOUR APPROACHMarker → cell → molecular function → disease or drug that involves it.

If you see… think…

Marker on all T cells → CD3

HIV receptor → CD4

EBV receptor → CD21

Rituximab target → CD20

Defect in X-linked hyper-IgM → CD40L

LPS co-receptor on monocytes → CD14

Reed-Sternberg cell markers → CD15, CD30

Absent CD18 with delayed cord separation → LAD type 1

Flow cytometry absent CD55/CD59 → PNH

Stem cell marker used for harvest → CD34