ImmunologyTier 1Medical Sciences concept

Immune cell surface marker identification (CD3, CD4, CD8, CD19, CD20, CD56)

Core concept

Marker	Cell	Note
CD34	Haematopoietic stem/progenitor cell	Used to enumerate and select cells for transplant (target >2 x 10^6 CD34+/kg); also endothelium
CD3	All T cells (~70% of lymphocytes)	Part of the TCR complex
CD4	Helper T cells (~60-70% of CD3+), also monocytes/macrophages	Binds MHC class II; HIV co-receptor
CD8	Cytotoxic T cells	Binds MHC class I; rises with viral infection
CD19	All B cells - earliest and latest B marker	Retained on plasmablasts; CAR-T target
CD20	Mature B cells	Absent on pro-B and on plasma cells - why rituximab does not deplete existing plasma cells or eliminate established autoantibody
CD16 / CD56	NK cells (CD3- CD56+)	CD16 = FcgammaRIII -> ADCC
CD14	Monocytes/macrophages	LPS co-receptor
CD15, CD30	Reed-Sternberg cells	See below
CD25	IL-2Ralpha - activated T cells, Tregs (CD4+CD25+FOXP3+)	Basiliximab target; soluble CD25 raised in HLH
CD38, CD138	Plasma cells	Daratumumab (CD38) target
CD52	Lymphocytes, monocytes	Alemtuzumab target
CD55 / CD59	GPI-anchored	Lost in PNH

CD4 count thresholds

Normal >500/microL
200-500 - moderate depletion; increasing risk of TB, bacterial pneumonia, herpes zoster, oral candidiasis
<200 - AIDS-defining; PJP prophylaxis indicated
<100 - toxoplasmosis, cryptococcus
<50 - CMV retinitis, disseminated MAC, CNS lymphoma, PML
Normal CD4:CD8 ratio ~2:1
- Reversed (<1) in HIV, acute viral infection (CMV, EBV), CVID, sarcoidosis (in blood; ratio is raised in sarcoid BAL fluid, usually >4)

Lineage assignment in haematological malignancy

Disease	Immunophenotype
CLL	CD5+ CD19+ CD23+, weak surface Ig, CD20 dim; CD5 is a T-cell marker aberrantly expressed - the hallmark
Mantle cell	CD5+ CD19+ CD23-, cyclin D1+ / t(11;14)
Follicular lymphoma	CD10+, BCL2+, t(14;18)
Classical Hodgkin lymphoma	*Reed-Sternberg: CD15+ CD30+, CD45-, and B-cell markers CD19/CD20/CD79a NEGATIVE* (PAX5 weakly +)
Nodular lymphocyte-predominant	CD20+ CD45+, CD15- CD30- - the reverse pattern
Anaplastic large cell lymphoma	CD30+, ALK+/-
Hairy cell leukaemia	CD11c, CD25, CD103, CD123, annexin A1
AML	CD13, CD33, CD117, MPO; CD34 on blasts
B-ALL	CD19, CD10 (CALLA), TdT
Myeloma	CD138+, CD38+, CD19-, cytoplasmic light chain restriction
PNH	Loss of CD55/CD59 on granulocytes and RBC; FLAER most sensitive

Clonality: B cells by kappa:lambda light chain restriction (normal ~2:1; a ratio >4:1 or <0.5:1 suggests clonality); T cells by TCR gene rearrangement or Vbeta repertoire

Antibody therapeutics are named by their target CD - knowing the marker predicts the toxicity
- Rituximab / obinutuzumab (CD20) - hypogammaglobulinaemia, hepatitis B reactivation, PML, blunted vaccine responses; plasma cells are CD20-negative, so pre-existing antibody titres persist
- Daratumumab (CD38) - *interferes with the indirect antiglobulin test and with blood cross-matching* (RBCs express CD38) - warn the blood bank and phenotype before starting
- Alemtuzumab (CD52) - profound prolonged lymphopenia, secondary autoimmunity (ITP, thyroid disease, anti-GBM)
- Blinatumomab (CD19 x CD3 bispecific), CAR-T (CD19, BCMA) - cytokine release syndrome and ICANS; CD19-negative relapse is the classic escape mechanism
- Brentuximab vedotin (CD30) - Hodgkin, ALCL; peripheral neuropathy
Flow cytometry practicalities - report absolute counts, not percentages alone; a normal CD4 percentage with lymphopenia still means a low absolute CD4
CD34 selection in stem cell grafts depletes T cells -> less GVHD but slower immune reconstitution and more infection/relapse
Aberrant marker expression is what identifies malignancy - CD5 on a B cell, CD30 on a large cell, or loss of a pan-T marker

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