B-Lymphoblastic Leukemia (B-ALL): Understanding Your Pathology Report

Section Editor: David Li MD
June 22, 2026

B-lymphoblastic leukemia, also called B-cell acute lymphoblastic leukemia (B-ALL), is a type of blood cancer that starts in the bone marrow, the soft inner part of bones where blood cells are made. In this disease, the bone marrow produces large numbers of immature white blood cells called lymphoblasts.

These lymphoblasts come from a type of white blood cell called a B cell. Healthy B cells normally help the body fight infection. In B-ALL, the abnormal lymphoblasts do not mature properly or work like normal B cells. Instead, they multiply quickly and crowd out healthy red blood cells, platelets, and other white blood cells. This leads to the symptoms and complications of leukemia.

This article will help you understand the findings in your pathology report for B-lymphoblastic leukemia, what each term means, and why it matters for your care or the care of your child.

What causes B-lymphoblastic leukemia?

The exact cause of B-lymphoblastic leukemia (B-ALL) is usually not known. Most cases arise when genetic changes accumulate in a single bone marrow cell over time, causing it to grow out of control as a leukemic lymphoblast. A few factors are known to increase the risk:

Inherited genetic conditions — Some conditions present from birth, such as Down syndrome, are linked with a higher risk. Most people with these conditions never develop leukemia.
Radiation exposure — Including earlier cancer treatment with radiation therapy.
Inherited gene variations — Common inherited differences in genes that control how blood cells grow and divide (such as ARID5B and IKZF1) can slightly raise the risk. These are not the same as a strong inherited cancer syndrome, and most people who carry them never develop leukemia.

For most patients, B-ALL has no clear cause, is not contagious, and cannot be prevented.

What are the symptoms of B-lymphoblastic leukemia?

The symptoms of B-lymphoblastic leukemia (B-ALL) happen because the leukemic lymphoblasts fill the bone marrow and prevent it from making enough healthy blood cells. Common symptoms include:

Fatigue, weakness, and shortness of breath — caused by a shortage of red blood cells (anemia).
Frequent infections — a shortage of normal white blood cells makes it harder to fight germs.
Easy bruising or bleeding — a shortage of platelets makes it harder to form blood clots.
Bone or joint pain — caused by the buildup of lymphoblasts inside the bones.
Swollen lymph nodes, liver, or spleen.
Fever, night sweats, or unexplained weight loss.

What is the difference between B-lymphoblastic leukemia and B-lymphoblastic lymphoma?

B-lymphoblastic leukemia (B-ALL) and B-lymphoblastic lymphoma (B-LBL) are very similar diseases that involve the same type of cancer cell. The difference is where the cancer cells are found:

B-ALL — Diagnosed when the cancer cells are present in the bone marrow and blood.
B-LBL — Diagnosed when the cancer cells are found mainly outside the bone marrow and blood, such as in the lymph nodes, liver, spleen, central nervous system, or skin.

Because of this overlap, the two are often described together as B-ALL/LBL and treated similarly.

How is the diagnosis made?

The diagnosis of B-lymphoblastic leukemia (B-ALL) is made by examining the blood and bone marrow. Doctors usually begin investigating when a complete blood count (CBC) shows abnormal numbers of lymphocytes or immature blasts. To confirm the diagnosis, a sample of the bone marrow is obtained via bone marrow aspiration and biopsy, usually from the back of the hip bone after the area is numbed. A pathologist examines the sample under the microscope and counts the proportion of lymphoblasts.

Flow cytometry is the main test used to confirm that the blasts are immature B cells. It measures proteins on and inside the cells, and in B-ALL the lymphoblasts typically carry B-cell markers such as CD19, CD22, and CD79a, along with markers of immaturity such as CD10, PAX5, and TdT. Immunohistochemistry, which also uses antibodies to detect specific proteins, may be used. Chromosome and molecular tests look for the genetic changes described later in this article. Finally, a sample of the fluid around the brain and spinal cord (cerebrospinal fluid) is often collected via lumbar puncture to assess whether leukemia cells have spread to the central nervous system.

What does B-lymphoblastic leukemia look like under the microscope?

Under the microscope, B-lymphoblastic leukemia (B-ALL) usually shows these features:

The bone marrow is packed with lymphoblasts that replace the normal blood-forming cells.
The lymphoblasts are larger than normal lymphocytes. Each has a large nucleus (the cell’s control center) that takes up most of the cell. The chromatin (genetic material in the nucleus) is fine and evenly spread, and small round structures called nucleoli may be seen inside the nucleus.
The cytoplasm (the part of the cell outside the nucleus) is thin and pale.
In blood smears, lymphoblasts may also be visible, often in very high numbers.

Because the blasts do not mature, very few normal B cells are present.

Genetic changes and molecular testing in B-lymphoblastic leukemia

B-lymphoblastic leukemia (B-ALL) is classified into subtypes based on the genetic changes found in leukemic lymphoblasts. Pathologists look for these changes using tests such as FISH (fluorescence in situ hybridization), PCR (polymerase chain reaction), and next-generation sequencing (NGS). These changes are among the most important findings in the report because they guide treatment and help predict how the leukemia will behave. Common patterns include:

ETV6::RUNX1 fusion and high hyperdiploidy — Hyperdiploidy means the cells have extra chromosomes. Both are linked with a more favorable outcome.
BCR::ABL1 fusion (Philadelphia chromosome) — This change joins two genes, BCR and ABL1, to form a new gene that drives leukemia growth, and it defines Philadelphia chromosome-positive B-ALL. Once considered high risk, its presence is now used to add a drug class called tyrosine kinase inhibitors to treatment, which block the abnormal protein and have greatly improved outcomes.
KMT2A rearrangement — More common in infants and generally linked with a higher risk of relapse, which may lead the team to consider more intensive treatment.
Hypodiploidy — Too few chromosomes; associated with a higher risk.
iAMP21 — Associated with a higher chance of relapse and usually treated with stronger therapy.
Philadelphia chromosome-like (Ph-like) B-ALL — The pattern of gene activity resembles Philadelphia chromosome-positive disease even though the BCR::ABL1 fusion is absent. Some of these cases may respond to targeted drugs.

Your report will describe any genetic changes that were found and may list the World Health Organization (WHO) subtype based on those changes.

Measurable residual disease (MRD)

After treatment for B-lymphoblastic leukemia (B-ALL) begins, very sensitive tests such as flow cytometry, PCR, or NGS are used to detect small numbers of leukemia cells that may remain. This is called measurable residual disease, also called minimal residual disease (MRD). The MRD result is one of the strongest predictors of whether the leukemia will return. It is used throughout treatment to measure how well the leukemia is responding and to help decide whether therapy should be intensified, immunotherapy added, or a stem cell transplant considered.

What is the prognosis for B-lymphoblastic leukemia?

Prognosis means the expected outcome of a disease. Unlike most solid tumors, B-lymphoblastic leukemia (B-ALL) is not assigned a stage based on size or spread because it begins in the bone marrow and blood rather than as a single mass. Instead, the likely outcome is estimated from a combination of factors:

Age — Children, especially those between 1 and 10 years old, generally do better than infants under 1 year and adults.
White blood cell count at diagnosis — A very high count is associated with a higher risk.
Genetic subtype — Changes such as high hyperdiploidy and ETV6::RUNX1 are favorable, while KMT2A rearrangement, hypodiploidy, and iAMP21 carry a higher risk.
Involvement of other sites — Leukemia cells found in the brain, spinal cord, or testes require additional treatment directed at those areas.
Response to treatment (MRD) — Reaching an MRD-negative result, meaning no leukemia can be detected by sensitive testing, is one of the most favorable signs.

Outcomes have improved substantially. Most children with B-ALL are now cured, with five-year survival above 90% for childhood disease. Outcomes in adults have historically been lower, but they have improved with modern chemotherapy combined with immunotherapy, reaching four-year survival of roughly 80 to 85% in younger adults who can receive these treatments. Philadelphia chromosome-positive B-ALL, once considered high risk, now reaches five-year survival above 80% when tyrosine kinase inhibitors are combined with chemotherapy or immunotherapy. Your prognosis depends on your own combination of these factors, which your care team can explain in the context of your specific report.

What happens after a diagnosis of B-lymphoblastic leukemia?

Once B-lymphoblastic leukemia (B-ALL) is confirmed, the care team plans treatment based on the identified genetic changes, the patient’s age, and overall risk. Treatment is usually given over about two to three years and is delivered in phases. The findings on the pathology report shape several decisions:

Combination chemotherapy — The backbone of treatment, given in phases (often called induction, consolidation, and maintenance) to bring the leukemia into remission and keep it there.
Treatment aimed at the brain and spinal cord — Because B-ALL can spread to the central nervous system, chemotherapy is given into the cerebrospinal fluid through a lumbar puncture, and radiation is sometimes used.
Tyrosine kinase inhibitors — For Philadelphia chromosome-positive B-ALL, drugs such as imatinib, dasatinib, or ponatinib are added to target the BCR::ABL1 protein.
Immunotherapy — Drugs such as blinatumomab (which directs the patient’s own T cells against the leukemia) and inotuzumab ozogamicin are increasingly used, including to clear measurable residual disease.
CAR T-cell therapy — For B-ALL that has come back or not responded, a patient’s own T cells can be re-engineered to attack the leukemia. Approved options exist for children and young adults as well as for older adults.
Stem cell (bone marrow) transplant — An allogeneic transplant, using blood-forming cells from a donor, may be considered for higher-risk or relapsed disease.

Throughout treatment, blood counts, repeat bone marrow biopsies, and MRD testing are used to assess how well the leukemia is responding, and the results guide decisions about whether more therapy is needed. Care is provided by a team that usually includes a hematologist or oncologist, specialized nurses, and supportive care specialists, and clinical trials are often available. Decisions about which of these approaches apply are made by the treatment team together with the patient or family, based on the specific findings in the report.

Questions to ask your doctor

What subtype of B-ALL do I have, and what does it mean for my prognosis?
What did the flow cytometry show about the markers on the leukemia cells?
Were any genetic changes found, such as the Philadelphia chromosome (BCR::ABL1), a KMT2A rearrangement, or a change in the number of chromosomes?
Am I (or is my child) considered standard risk or high risk, and what does that mean?
Has the leukemia spread to the cerebrospinal fluid, central nervous system, or testes?
Will I need treatment directed at the brain and spinal cord?
What treatment phases should I expect, and how long will treatment last?
Could a tyrosine kinase inhibitor, immunotherapy, or CAR T-cell therapy be part of my treatment?
How will measurable residual disease (MRD) be tested, and how will the results affect my treatment?
Might I need a stem cell (bone marrow) transplant?
How do treatment options differ for children and adults?
Are there clinical trials that I should consider?