PBMC Isolation Protocol: Density Gradient Centrifugation with a Lymphocyte Separation Medium

Peripheral blood mononuclear cells (PBMCs) – lymphocytes and monocytes – are the starting material for immunophenotyping, T-cell assays, ELISpot, cytokine release assays and many cell therapy workflows. The most widely used way to isolate them is density gradient centrifugation over a lymphocyte separation medium at 1.077 g/mL.

This protocol describes the standard procedure for fresh, anticoagulated human whole blood using Lymphobio, a ready-to-use lymphocyte separation solution at 1.077 g/mL.

How density gradient separation works

The separation medium is a polysucrose-based solution adjusted to a density of 1.077 g/mL at 20 °C. During centrifugation:

• Erythrocytes and granulocytes are denser than 1.077 g/mL. They sediment through the medium to the bottom of the tube. Erythrocytes aggregate in the medium, which speeds up their sedimentation.
• Mononuclear cells (lymphocytes, monocytes) are less dense. They collect as a whitish band at the interface between plasma and medium.
• Plasma and most platelets stay in the upper layer.

The method depends on correct density, temperature and an undisturbed gradient. Most failed isolations go back to one of these three.

Materials

• Fresh human whole blood with anticoagulant (EDTA, sodium heparin or citrate)
• Lymphobio lymphocyte separation solution, 1.077 g/mL, equilibrated to 18–20 °C
• Sterile PBS without Ca²⁺/Mg²⁺ (optionally with 2 mM EDTA and/or 0.5–2 % serum or albumin for washing)
• Sterile 15 mL or 50 mL conical tubes, serological pipettes, Pasteur pipettes
• Centrifuge with swing-out rotor, temperature control and the option to switch off the brake
• Trypan blue and a cell counter or haemocytometer
• Biosafety cabinet (class II) and appropriate PPE

Biosafety: Treat all human blood as potentially infectious and work under BSL-2 conditions according to your institution’s rules.

Step-by-step protocol

1. Prepare reagents

1. Bring the separation medium, PBS and blood to room temperature (18–20 °C). The density of the medium is specified at 20 °C. Colder or warmer medium changes the separation.
2. Mix the separation medium gently by inverting the bottle. Work aseptically.

2. Dilute the blood

3. Dilute the blood 1:1 with PBS (e.g. 15 mL blood + 15 mL PBS) and mix gently. Dilution reduces cell aggregation and trapping of lymphocytes in the red cell pellet.

3. Layer the blood onto the medium

4. Add the separation medium to a conical tube:
   • 50 mL tube: 15 mL medium
   • 15 mL tube: 3–4 mL medium

5. Hold the tube at an angle and slowly overlay the diluted blood onto the medium. Run it down the tube wall so that the two phases do not mix:

   • 50 mL tube: up to 30–35 mL diluted blood
   • 15 mL tube: up to 8 mL diluted blood

   A sharp boundary between the red layer and the clear medium should remain visible.

4. Centrifuge

6. Centrifuge at 400 × g for 30–40 minutes at 18–20 °C in a swing-out rotor.
   • Acceleration: low
   • Brake: off. A brake disturbs the interface and mixes the layers.

5. Harvest the PBMC layer

7. After centrifugation you will see, from top to bottom:
   • plasma
   • a whitish, cloudy PBMC band
   • clear separation medium
   • a red pellet of erythrocytes and granulocytes
8. Remove part of the upper plasma layer with a pipette without touching the PBMC band. If you need autologous plasma, keep it.
9. Collect the PBMC band with a sterile Pasteur pipette and transfer it to a new tube. Take as little separation medium as possible; residual medium lowers viability and makes washing less effective.

6. Wash the cells

10. Add at least 3 volumes of PBS to the harvested cells (e.g. fill up to 40–50 mL in a 50 mL tube) and mix gently.
11. Centrifuge at 300 × g for 10 minutes at room temperature, brake on. Discard the supernatant.
12. Resuspend the pellet in PBS and centrifuge again at 200 × g for 10–15 minutes. The lower speed removes most remaining platelets with the supernatant.
13. Optional: if the pellet is visibly red, perform a short red cell lysis with an ammonium chloride-based buffer and wash once more.

7. Count and assess viability

14. Resuspend the cells in culture medium or buffer and count with trypan blue.
    • Viability: typically above 90–95 % for fresh blood from healthy donors.
    • Yield: typically 0.5–2 × 10⁶ PBMCs per mL of whole blood. It depends on the donor, the anticoagulant and the time to processing.

8. Use or cryopreserve

15. Use the cells immediately or cryopreserve them:
    • Freezing medium: 90 % serum with 10 % DMSO. For workflows that should avoid bovine material, use human serum or human AB serum instead of FBS.
    • Freezing: slowly, at about −1 °C/min.
    • Storage: in liquid nitrogen vapour phase.

Culture after isolation

For T-cell, NK-cell and monocyte assays, many labs use human serum rather than FBS. It avoids xenogeneic background stimulation.

• AB serum: Type AB serum contains no anti-A or anti-B isoagglutinins and can be used with cells from donors of every blood group. See human serum for PBMCs and immunoassays and the PBMC & Immunology application page.
• Heat-inactivated serum: If complement activity would interfere with your assay, use heat-inactivated human serum.

Troubleshooting

Variations

• Buffy coats and leukapheresis products contain far more cells per mL. Dilute them more strongly (often 1:2 to 1:4 with PBS) before layering, so the gradient is not overloaded.
• Separation tubes with a porous insert let you add blood without careful layering. Follow the tube manufacturer’s instructions for volumes, speed and brake settings.
• Other species: 1.077 g/mL is optimised for human cells. Mouse and some other species need a different density.

FAQ

What density should a lymphocyte separation medium have for human PBMCs? 1.077 g/mL at 20 °C. This is the standard density for separating human mononuclear cells from granulocytes and erythrocytes.

Are polysucrose-based density gradient media interchangeable? Polysucrose-based density gradient media at 1.077 g/mL all use the same principle and the same protocol. Lymphobio is designed as a direct replacement in existing protocols.

Why must the centrifuge brake be off? The deceleration of a braking rotor stirs up the interface. The PBMC band then mixes with medium and red cells, and yield and purity drop.

Which anticoagulant is best for PBMC isolation? EDTA, heparin and citrate all work. Choose according to your downstream assay. Heparin, for example, can interfere with some PCR-based methods. EDTA blood is common for flow cytometry.

How long can blood be stored before PBMC isolation? Process it as soon as possible, ideally within a few hours at room temperature. With longer storage, granulocytes change density and contaminate the PBMC fraction, and viability decreases.

Can I freeze PBMCs in human serum instead of FBS? Yes. Many immunology labs use human serum or human AB serum with 10 % DMSO to avoid bovine proteins in later assays.

Order Lymphobio

Lymphobio is available in 100 mL and 500 mL, with bulk volumes on request. Each lot comes with a CoA including density, osmolality, endotoxin and sterility. Prices and availability on request – request a quote.

Related: PBMC & Immunology · Human Serum Type AB · Human Serum for PBMCs & Immunoassays · Cell Dissociation & Separation Solutions