B cell and plasma cell !
- Cell
B cells recognize antigens with surface antibody; when activated they become plasma cells, factories that secrete antibody, or memory cells.
Updated 2026-11-01
What it is
B cells are lymphocytes that mature in the bone marrow and carry a surface antibody (the B-cell receptor, IgM and IgD on naive cells). After meeting antigen, and usually with help from CD4 T cells, they multiply in germinal centers, refine their antibodies and switch class. They end as plasma cells, which secrete large amounts of antibody, or as long-lived memory B cells; they also present antigen to T cells.
Key facts
- Origin: lymphoid progenitor; matures in the bone marrow, where strongly self-reactive B cells are deleted or made anergic.
- Memory B cells last for years; many long-lived plasma cells return to the bone marrow.
- B-1 and marginal zone B cells make quick IgM against bacterial polysaccharides.
Step by step
- A naive B cell binds antigen with its B-cell receptor in a lymphoid follicle.
- It internalizes the antigen and presents peptides on MHC class II to a helper T cell.
- CD40L and cytokines from the T cell activate it fully.
- In the germinal center it mutates its antibody genes and the best binders are selected; class switching occurs.
- It becomes a plasma cell that secretes antibody or a memory B cell.
Markers
- CD19
- CD20
- CD21 (CR2, EBV receptor)
- CD40
- Surface IgM/IgD
- CD138 (plasma cell)
Clinical notes
- X-linked agammaglobulinemia (BTK): no mature B cells, very low antibodies, bacterial infections in boys after maternal antibodies fade.
- Common variable immunodeficiency: low antibodies with recurrent sinus and lung infections, usually diagnosed in adolescents or adults.
- Multiple myeloma: malignant plasma cells make a monoclonal protein (M spike) and damage bone, kidney and marrow.
- Chronic lymphocytic leukemia and most non-Hodgkin lymphomas are B-cell cancers; anti-CD20 antibodies target them.
In the lab
- Serum immunoglobulin levels (IgG, IgA, IgM) and vaccine antibody responses test B-cell function.
- Flow cytometry counts CD19 or CD20 cells.
- Protein electrophoresis and immunofixation detect a monoclonal protein.
Test yourself
Think of the answer, then tap the question to see it.
Which surface markers are classic for B cells?
CD19, CD20 and CD21, together with surface immunoglobulin and MHC class II.
What two processes in the germinal center improve the antibody response?
Somatic hypermutation with selection (affinity maturation) and class switch recombination.
Which gene is defective in X-linked agammaglobulinemia, and what is missing?
BTK (Bruton tyrosine kinase); mature B cells and antibodies are missing.
How does a plasma cell look under the microscope?
An eccentric nucleus with clumped chromatin, a pale perinuclear Golgi zone and basophilic cytoplasm full of rough endoplasmic reticulum.
Related
- B-cell activation, class switching and affinity maturation
- Antibody structure and isotypes (IgG, IgA, IgM, IgE, IgD)
- CD4⁺ helper T cell
- Lymph node: cortex, paracortex, medulla
- Bone marrow and hematopoiesis of immune cells
- B-cell (antibody) deficiencies: XLA, CVID, IgA deficiency, hyper-IgM
- CD markers: the table to know
Related conditions
Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.
Sources: 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); B Cells (British Society for Immunology, 2024); Multiple myeloma (MedlinePlus, U.S. National Library of Medicine, 2024).
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Representative model, for educational purposes only.