[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Immunology](https://durgical.com/en/immunology/) › [Immune cells](https://durgical.com/en/immunology/#a-cells) › B cell and plasma cell [Immunology](https://durgical.com/en/immunology/) · [Immune cells](https://durgical.com/en/immunology/#a-cells) # 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 ## Key points - Markers: CD19, CD20, CD21 (receptor used by Epstein-Barr virus), CD40 and MHC class II; plasma cells lose CD20 and express CD138. - Naive B cells express surface IgM and IgD. - Germinal centers: somatic hypermutation (affinity maturation) and class switching to IgG, IgA or IgE; switching needs CD40-CD40L. - Protein antigens are T-dependent; repeating polysaccharides can activate B cells without T cells (T-independent, mainly IgM). - Plasma cell: eccentric clock-face nucleus, pale perinuclear Golgi zone and abundant rough endoplasmic reticulum. ## 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. ## Memory aid The B cell goes to Boot camp in the follicle: it trains its aim (affinity), changes uniform (class switch) and graduates as a sharpshooter (plasma cell) or a veteran (memory). ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/immunology/b-cell-and-plasma-cell/) ## Related - [B-cell activation, class switching and affinity maturation](https://durgical.com/en/immunology/b-cell-activation-class-switching-and-affinity-maturation/) - [Antibody structure and isotypes (IgG, IgA, IgM, IgE, IgD)](https://durgical.com/en/immunology/antibody-structure-and-isotypes/) - [CD4⁺ helper T cell](https://durgical.com/en/immunology/cd4-helper-t-cell/) - [Lymph node: cortex, paracortex, medulla](https://durgical.com/en/immunology/lymph-node-cortex-paracortex-medulla/) - [Bone marrow and hematopoiesis of immune cells](https://durgical.com/en/immunology/bone-marrow-and-hematopoiesis-of-immune-cells/) - [B-cell (antibody) deficiencies: XLA, CVID, IgA deficiency, hyper-IgM](https://durgical.com/en/immunology/b-cell-deficiencies-xla-cvid-iga-deficiency-hyper-igm/) - [CD markers: the table to know](https://durgical.com/en/immunology/cd-markers-the-table-to-know/) ## Related conditions - [Multiple myeloma](https://durgical.com/en/conditions/multiple-myeloma/) [← All of immunology](https://durgical.com/en/immunology/) 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](https://openstax.org/books/anatomy-and-physiology-2e/pages/21-4-the-adaptive-immune-response-b-lymphocytes-and-antibodies) (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); [B Cells](https://www.immunology.org/public-information/bitesized-immunology/cells/b-cells) (British Society for Immunology, 2024); [Multiple myeloma](https://medlineplus.gov/ency/article/000583.htm) (MedlinePlus, U.S. National Library of Medicine, 2024). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/immunology/b-cell-and-plasma-cell/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/b-cell-and-plasma-cell/