Hematology & Immunology & Internal Medicine
Plasma cells
Plasmocytes—also known as plasma cells or immunocytes—are the terminal effector cells of the B-cell lineage, specialized in the synthesis and massive secretion of immunoglobulins (antibodies) destined for the bloodstream and mucosal secretions. They result from the final differentiation of naive B lymphocytes activated by encountering a specific antigen in the secondary lymphoid organs (lymph nodes, spleen, Peyer’s patches)—a differentiation that occurs in two phases: a rapid initial phase of proliferation in the T-zone of lymphoid organs, giving rise to short-lived plasmablasts that rapidly produce low-affinity antibodies (primary response); and a second, prolonged phase within the follicular germinal centers where isotype switching (switch from IgM to IgG, IgA, or IgE) and affinity maturation occur through somatic hypermutation of the genes in the variable regions of immunoglobulins — resulting in long-lived plasma cells that migrate to the bone marrow to reside in specialized niches and ensure continuous production of high-affinity antibodies for decades, forming the humoral immune memory. Morphologically, the plasma cell is recognizable on bone marrow smears or tissue histological sections by its characteristic appearance: a medium- to large-sized cell with an off-center nucleus containing chromatin condensed into «spoke-like or »clock face,« abundant, strongly basophilic cytoplasm (rich in rough endoplasmic reticulum dedicated to immunoglobulin synthesis), and a clear perinuclear zone corresponding to the Golgi apparatus. These morphological characteristics allow them to be distinguished from mature B lymphocytes (central nucleus + scant cytoplasm), activated B lymphocytes, and plasmablasts (intermediate morphology). The normal proportion of plasma cells in the bone marrow is less than 5% of total bone marrow cell count—bone marrow plasmacytosis exceeding 10% is one of the major diagnostic criteria for multiple myeloma, a malignant plasma cell disorder that is the second most common hematologic malignancy after non-Hodgkin lymphomas, with approximately 3,500 new cases per year in Canada.
Biology and Immune Role
- Differentiation and maturation: Naive B lymphocyte → antigen activation + follicular helper T cell + co-stimulatory signals → short-lived plasmablast (rapid primary response + predominantly IgM) → maturation in germinal centers → isotype switching → affinity maturation → long-lived plasma cell → migration into bone marrow → specialized niches (stromal cell contact + CXCL12 + IL-6 + APRIL + BAFF) → long-term survival + continuous secretion of high-affinity antibodies + bone marrow plasma cell lifespan: weeks to decades
- Immunoglobulin production Each plasma cell produces antibodies of a single specificity (one antigen) and a single isotype (IgG + IgA + IgM + IgD + IgE) + secretion rate: 2,000 to 20,000 immunoglobulin molecules per second per plasma cell + the hypertrophied rough endoplasmic reticulum and developed Golgi apparatus reflect this intense secretory activity + immunoglobulins are composed of two heavy (H) chains and two light (L — kappa or lambda) chains linked by disulfide bridges + each plasma cell expresses either kappa light chains (normal kappa/lambda ratio ≈ 2/1) or lambda light chains, never both
- Immunophenotypic markers: CD138 (syndecan-1)—most characteristic and specific marker of mature plasma cells + CD38++ + CD19- (loses expression during terminal differentiation) + CD20- (unlike B lymphocytes) + CD45 variable + MUM1/IRF4+ (essential transcription factor for plasma cell differentiation) + Ig+ cytoplasm (intracellular immunoglobulin)
Plasma cell dyscrasias - clinical spectrum
| Entity | Definition and diagnostic criteria | Support and prognosis |
|---|---|---|
| Monoclonal gammopathy of undetermined significance (MGUS) | Serum monoclonal peak < 30 g/L + bone marrow plasma cells < 10 × 10³/L + absence of CRAB criteria (hypercalcemia + renal failure + anemia + bone lesions) + absence of amyloidosis or other symptomatic plasma cell dyscrasia + benign entity but mandatory precursor of myeloma | Watchful waiting + risk of progression to multiple myeloma or IgM lymphoma: 1 % per year (low but cumulative risk — 10 % over 10 years) + follow-up CBC + electrophoresis + free light chains every 6–12 months + no treatment unless progression + prevalence: 3 % after age 50 + 5 % after age 70 |
| Symptomatic multiple myeloma (MM) | Clonal plasma cells in the bone marrow ≥ 10 × 10³/L and at least one CRAB criterion (Calcium > 2.75 mmol/L + Renal GFR < 40 mL/min or creatinine > 177 µmol/L + Anemia Hb < 100 g/L + Bone lesions) OR biomarker of malignancy: plasma cells ≥ 60% + free light chains involved/uninvolved ≥ 100 + > 1 focal lesion on MRI | Systemic treatment: induction (bortezomib + lenalidomide + dexamethasone — VRd) + consolidation by autologous hematopoietic stem cell transplantation (ASCT) in eligible patients + maintenance (lenalidomide) + new therapies: daratumumab (anti-CD38) + isatuximab + carfilzomib + new lines: BCMA-targeted baby CAR-T secretion + median overall survival: 5–7 years in 2024 vs < 3 years before 2000 |
| Asymptomatic multiple myeloma (smoldering myeloma) | Serum monoclonal peak ≥ 30 g/L and/or bone marrow plasma cells ≥ 10% + absence of CRAB criteria and biomarkers of malignancy → intermediate zone between MGUS and symptomatic MM | Active surveillance without treatment in most cases + risk of progression to MM: 10 % cases per year in the early years + high-risk patients (plasma cells ≥ 60 × 10³/L + light chains > 100 + abnormal MRI) → early treatment considered in clinical trials + CBC + electrophoresis + whole-body MRI every 3–6 months |
| Solitary plasmacytoma | Single bone plasmacytic lesion (solitary bone plasmacytoma) or extra-osseous lesion (extramedullary plasmacytoma) + normal bone marrow (< 10% plasma cells) + absence of CRAB criteria + absent or low serum monoclonal peak | Curative-intent local radiation therapy → 45–50 Gy to the lesion + local control rate 90% (%) + risk of progression to MM: 50% % at 10 years for bone plasmacytoma + 30% % for extraosseous plasmacytoma + close monitoring with serial electrophoresis and MRI |
| Plasma cell leukemia | Circulating plasma cells > 2 × 10⁹/L in the blood + or > 20% of circulating leukocytes + primary (de novo) or secondary (progression from MM) form + aggressive course | Very aggressive hematologic malignancy with a poor prognosis (median survival < 12–18 months) requiring intensive induction therapy, followed by ASCT if eligible, and novel therapies (daratumumab + CAR-T). |
| Light chain amyloidosis (AL amyloidosis) | Tissue deposition of monoclonal immunoglobulin fibrils (primarily light chains) in organs → renal + cardiac + hepatic + neurological dysfunction + risk of organ failure + associated with a small clonal plasma cell proliferation (often < 10 % bone marrow plasma cells) | Treatment: daratumumab + bortezomib + cyclophosphamide + dexamethasone (Dara-CyBorD) ± ASCT + rapid treatment is vital (amyloid heart can progress very rapidly to sudden death) + abdominal fat or rectal biopsy for histological diagnosis |
CRAB criteria for multiple myeloma
- C — Calcium (hypercalcemia): Corrected serum calcium > 2.75 mmol/L OR > 0.25 mmol/L above the upper limit of normal. Mechanism: Secretion of RANK-L by myeloma cells → osteoclast activation → bone resorption → calcium release. Symptoms: Polyuria, polydipsia, constipation, confusion, nausea, anorexia. Treatment: Hyperhydration, IV bisphosphonates (zoledronic acid).
- R - Renal (kidney failure): creatinine > 177 µmol/L or GFR < 40 mL/min + multiple mechanisms: myeloma cast nephropathy (light chains precipitate in the tubules with Tamm-Horsfall protein) + renal amyloidosis + hypercalcemia + dehydration + nephrotoxic agents (NSAIDs + iodinated contrast agents to be avoided!) + light chain deposition + cast nephropathy is often reversible with treatment
- Anemia: Hb 20 g/L below normal value + mechanism: replacement of hematopoietic bone marrow by myeloma plasma cells + inhibition of erythropoiesis by tumor cytokines (IL-6) + associated renal failure (EPO deficiency) + most often normocytic normochromic anemia + treatment: transfusional support + EPO + myeloma treatment
- B — Bone (bone lesions): lytic lesions (cavities) visible on X-rays or CT scans of the entire skeleton (skull, spine, ribs, pelvis, and long bones) + osteoporosis + vertebral compression fractures + pathological fractures of the long bones + mechanism: activation of osteoclasts + inhibition of osteoblasts by cytokines from the tumor microenvironment + IV bisphosphonates (denosumab or zoledronic acid) reduce bone events by 40%
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The initial assessment for suspected plasma cell dyscrasias includes: serum protein electrophoresis (SPEP) with immunofixation if a monoclonal peak is detected + serum free light chain assay (free kappa and lambda light chains + kappa/lambda ratio) + complete blood count (CBC) + creatinine + serum calcium + LDH + beta-2 microglobulin + whole-body skeletal survey (skull + spine + pelvis + long bones) or whole-body MRI (more sensitive) + bone marrow aspirate with immunophenotyping and karyotyping. A ratio of involved/uninvolved free light chains ≥ 100 is a sufficient criterion for malignancy to initiate treatment even in the absence of CRAB criteria.
Situations Requiring Urgent Hematologic Consultation
Consult the emergency department or a hematologist within the following days if an electrophoresis reveals a significant serum monoclonal peak, associated with unexplained normochromic normocytic anemia and diffuse bone pain (spine + ribs + pelvis), or if hypercalcemia is discovered without any other identifiable cause. These signs constitute a classic presentation of multiple myeloma requiring an urgent hematological workup including free light chains, bone marrow aspiration and biopsy, skeletal imaging, and prompt initiation of treatment if confirmed. Acute back pain in an elderly patient with hypercalcemia and anemia is a red flag for myeloma.
For the investigation of an abnormal serum protein electrophoresis, the initial workup for suspected plasma cell dyscrasia, and referral to hematology, Clinique Omicron offers medical consultations at its service points in Quebec and via telemedicine. To book an appointment, visit cliniqueomicron.ca.
Consult at Clinique Omicron
Clinique Omicron's physicians and nurse practitioners (NPs) prescribe and interpret serum protein electrophoresis and free light chains when suspecting a plasma cell dyscrasia (bone pain + unexplained anemia + hypercalcemia + kidney failure of undetermined origin), manage patients with MGUS (serial electrophoresis + CBC + annual creatinine), and promptly refer symptomatic cases requiring a bone marrow biopsy and treatment to hematology. Consultations are available at several service locations in Quebec and through telemedicine. To book an appointment, visit cliniqueomicron.ca.
The content of this page is for informational purposes only and does not replace the advice of a doctor or hematologist. Multiple myeloma is a malignant blood disorder that requires specialized management by a hematology team for diagnosis and treatment. A complete evaluation should be initiated quickly in the event of any suspicion of symptomatic plasma cell dyscrasia.
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