[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Immunology](https://durgical.com/en/immunology/) › [Adaptive immunity](https://durgical.com/en/immunology/#a-adaptive) › B-cell activation, class switching and affinity maturation [Immunology](https://durgical.com/en/immunology/) · [Adaptive immunity](https://durgical.com/en/immunology/#a-adaptive) # B-cell activation, class switching and affinity maturation - Process B cells that recognize a germ get help from T cells, then improve their antibodies and change their type in lymph nodes. Updated 2026-11-01 ## Key points - CD40L (T cell) plus CD40 (B cell) is required for class switching. - AID drives both somatic hypermutation and class switching. - Affinity maturation happens in germinal centers, selected by follicular dendritic cells. - Class switching changes the constant region, not the specificity. - Conjugate vaccines make polysaccharide antigens T-dependent, so infants respond and develop memory. - X-linked hyper-IgM: CD40L defect, high IgM, very low IgG/IgA/IgE. ## What it is A naive B cell (surface IgM and IgD) is activated when its receptor binds antigen. For protein antigens it also needs help from a follicular helper T cell that recognizes a peptide from the same antigen; this help, through CD40L and cytokines, starts the germinal center reaction. There the B cell mutates its antibody genes, the best binders are selected (affinity maturation), the constant region is changed (class switching), and plasma cells and memory B cells are produced. ## Step by step - A naive B cell binds native antigen with surface IgM/IgD (signal 1). - It internalizes the antigen and presents peptides on MHC class II. - A Tfh cell recognizing the same antigen binds; CD40L on the T cell engages CD40 on the B cell (signal 2), with IL-21 and other cytokines. - Activated B cells form a germinal center in the lymphoid follicle and proliferate. - Activation-induced cytidine deaminase (AID) causes somatic hypermutation of the variable region. - Follicular dendritic cells display antigen; B cells with higher affinity survive (affinity maturation), the rest die. - AID also drives class switch recombination: the constant region changes from IgM to IgG, IgA or IgE; specificity stays. - Cells exit as long-lived plasma cells (many go to the bone marrow) or memory B cells. ## T-dependent versus T-independent antigens | Feature | T-dependent | T-independent | | --- | --- | --- | | Typical antigen | Proteins | Polysaccharides, LPS (repeating units) | | T-cell help | Required (Tfh, CD40L) | Not required (extensive BCR cross-linking) | | Antibody | Class switched (IgG, IgA, IgE), high affinity | Mainly IgM, low affinity | | Memory | Yes | Little or none | | In children under two years | Good response | Poor response | ## Clinical notes - Hyper-IgM syndrome (most often X-linked CD40L defect): no class switching, normal or high IgM, low IgG, IgA and IgE, and opportunistic infections such as Pneumocystis. - Conjugate vaccines link a polysaccharide to a protein carrier, turning a T-independent response into a T-dependent one with memory (Haemophilus influenzae type b, pneumococcus, meningococcus). - The spleen is important for T-independent responses to encapsulated bacteria. ## Memory aid In the germinal center the B cell goes to school: it practices (hypermutation), takes exams (selection by follicular dendritic cells) and graduates in a new uniform (class switching). ## Test yourself Think of the answer, then tap the question to see it. ## Which interaction between T and B cells is essential for class switching? CD40 ligand (CD40L, CD154) on the activated T cell binding CD40 on the B cell, together with cytokines. ## Which enzyme is required for both somatic hypermutation and class switch recombination? Activation-induced cytidine deaminase (AID). ## Why do pure polysaccharide vaccines work poorly in children under two years of age? Polysaccharides are T-independent antigens; young children respond poorly to them and memory is minimal. Conjugating them to a protein recruits T-cell help. ## What is the antibody pattern in X-linked hyper-IgM syndrome? Normal or high IgM with very low IgG, IgA and IgE, because B cells cannot switch class without CD40L. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/immunology/b-cell-activation-class-switching-and-affinity-maturation/) ## Related - [Antibody structure and isotypes (IgG, IgA, IgM, IgE, IgD)](https://durgical.com/en/immunology/antibody-structure-and-isotypes/) - [Th1, Th2, Th17, Tfh and Treg](https://durgical.com/en/immunology/th1-th2-th17-tfh-and-treg/) - [Primary vs secondary response and memory](https://durgical.com/en/immunology/primary-vs-secondary-response-and-memory/) - [Antigen processing and presentation](https://durgical.com/en/immunology/antigen-processing-and-presentation/) - [B cell and plasma cell](https://durgical.com/en/immunology/b-cell-and-plasma-cell/) - [Lymph node: cortex, paracortex, medulla](https://durgical.com/en/immunology/lymph-node-cortex-paracortex-medulla/) - [Vaccine types: live, inactivated, subunit, toxoid, mRNA, vector](https://durgical.com/en/immunology/vaccine-types-live-inactivated-subunit-toxoid-mrna-vector/) - [B-cell (antibody) deficiencies: XLA, CVID, IgA deficiency, hyper-IgM](https://durgical.com/en/immunology/b-cell-deficiencies-xla-cvid-iga-deficiency-hyper-igm/) - [Spleen: white pulp, red pulp, asplenia](https://durgical.com/en/immunology/spleen-white-pulp-red-pulp-asplenia/) ## Microorganisms - [Streptococcus pneumoniae](https://durgical.com/en/microbiology/streptococcus-pneumoniae/) - [Haemophilus influenzae](https://durgical.com/en/microbiology/haemophilus-influenzae/) [← 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: [18.4 B Lymphocytes and Humoral Immunity](https://openstax.org/books/microbiology/pages/18-4-b-lymphocytes-and-humoral-immunity) (OpenStax (Microbiology), 2016); [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 (Anatomy and Physiology 2e), 2022); [Chapter 1: Principles of Vaccination](https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-1-principles-of-vaccination.html) (Centers for Disease Control and Prevention (CDC), Pink Book, 2021). ## **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-activation-class-switching-and-affinity-maturation/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/b-cell-activation-class-switching-and-affinity-maturation/