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

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.

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.

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Microorganisms

← All of 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 (OpenStax (Microbiology), 2016); 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies (OpenStax (Anatomy and Physiology 2e), 2022); Chapter 1: Principles of Vaccination (Centers for Disease Control and Prevention (CDC), Pink Book, 2021).

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Representative model, for educational purposes only.