18.4 B Lymphocytes and Humoral Immunity

B Lymphocyte Development and Activation

Development of B cells

B cells originate from hematopoietic stem cells in the bone marrow. These stem cells differentiate into common lymphoid progenitor cells, which then become pro-B cells, the earliest committed stage of B cell development.

Pro-B cells undergo V(D)J recombination, where variable (V), diversity (D), and joining (J) gene segments are rearranged to generate unique B-cell receptors (BCRs). This recombination is what creates the enormous diversity in antigen recognition across your B cell population. Successful recombination produces pre-B cells that express a functional pre-BCR and surface IgM.

Pre-B cells then differentiate into immature B cells, which face a critical checkpoint: negative selection. Immature B cells that bind strongly to self-antigens are either eliminated through apoptosis or undergo receptor editing to change their specificity. This step prevents autoimmune reactions by weeding out B cells that would attack your own tissues.

Immature B cells that survive negative selection migrate to secondary lymphoid organs (spleen, lymph nodes), where they mature into naïve B cells expressing both surface IgM and IgD as antigen receptors. These naïve B cells are now ready to encounter foreign antigen for the first time.

B-cell vs T-cell receptors

BCRs and TCRs both recognize antigens, but they do so in fundamentally different ways.

• B-cell receptors (BCRs) are membrane-bound immunoglobulins (antibodies) composed of two heavy chains and two light chains in a Y-shaped structure. The variable regions of the heavy and light chains form the antigen-binding site. BCRs recognize and bind to specific epitopes on native (unprocessed) antigens, including soluble proteins, carbohydrates, and lipids.
• T-cell receptors (TCRs) are heterodimeric proteins, most commonly composed of an α chain and a β chain (a small subset uses γ and δ chains). The variable regions of the α and β chains form the antigen-binding site. TCRs can only recognize processed peptide antigens presented by MHC molecules on antigen-presenting cells (APCs).

B Cell Activation and Antibody Response

T-dependent vs T-independent activation

T-dependent activation requires cooperation with helper T cells and is the more common pathway for protein antigens:

1. A B cell binds antigen through its BCR, then internalizes and processes it.
2. The B cell presents processed antigen peptides on MHC class II molecules to a helper T cell that recognizes the same antigen.
3. The activated helper T cell provides co-stimulatory signals (via CD40L binding to CD40) and secretes cytokines that drive B cell activation and differentiation.

T-dependent activation leads to germinal center formation, affinity maturation, class switching, and memory B cell development. This is why protein-based vaccines generate long-lasting immunity.

T-independent activation occurs without helper T cells and is triggered by non-protein antigens:

• Type 1 T-independent antigens (e.g., lipopolysaccharide) activate B cells directly through pattern recognition receptors like Toll-like receptors (TLRs).
• Type 2 T-independent antigens (e.g., bacterial capsular polysaccharides) have repetitive structures that crosslink multiple BCRs simultaneously, providing a strong activation signal.

T-independent responses produce antibodies quickly, but they are primarily IgM and do not generate memory B cells. This is why polysaccharide vaccines tend to provide shorter-lived protection compared to protein-conjugate vaccines.

Primary vs secondary antibody responses

The primary response occurs on first exposure to an antigen:

• There is a lag phase of 7–10 days while naïve B cells are activated and begin proliferating.
• The response predominantly produces IgM antibodies with relatively low affinity.
• Memory B cells specific to the antigen are generated during this response.

The secondary response occurs on subsequent exposure to the same antigen:

• The lag phase is much shorter (1–3 days) because memory B cells are already primed.
• The response predominantly produces high-affinity IgG (along with some IgA and IgE, depending on the tissue).
• Memory B cells rapidly differentiate into antibody-secreting plasma cells, producing higher antibody titers than the primary response.

Clonal Selection and Affinity Maturation

Clonal selection explains how the immune system amplifies the right B cells for the job. When an antigen enters the body, only B cells whose BCRs bind that antigen become activated. Those selected B cells proliferate, producing clones that differentiate into antibody-secreting plasma cells and memory B cells. B cells that don't recognize the antigen remain inactive.

Affinity maturation refines antibody quality over time. Inside germinal centers, activated B cells undergo somatic hypermutation, which introduces random point mutations in the variable regions of their antibody genes. B cells that happen to gain mutations improving their antigen-binding affinity are preferentially selected for survival, while those with lower affinity die off. The result is that antibodies become progressively better at binding their target with each round of selection.

Class switching (also called isotype switching) allows a B cell to change the constant region of its antibody heavy chain, switching from IgM to IgG, IgA, or IgE, while keeping the same antigen-binding specificity. Different isotypes serve different functions: IgG is the most abundant in blood and is excellent at opsonization, IgA protects mucosal surfaces, and IgE is involved in allergic responses and defense against parasites. Cytokines from helper T cells direct which class a B cell switches to.

Complement System and Effector Functions

Antibodies don't just bind to pathogens; they recruit other immune mechanisms to destroy them.

• Classical complement activation: When antibodies (especially IgG and IgM) bind to a pathogen surface, they can activate the classical complement pathway. This triggers a cascade of complement proteins that punch holes in pathogen membranes (via the membrane attack complex), recruit inflammatory cells, and enhance phagocytosis.
• Opsonization: Antibodies coating a pathogen act as molecular flags. Phagocytes like macrophages and neutrophils have Fc receptors that bind the constant region of antibodies, making it much easier for them to recognize, engulf, and destroy the antibody-coated pathogen.
• Neutralization: Antibodies can block a pathogen's ability to infect cells by binding to surface molecules the pathogen needs for attachment or entry. This is how many vaccine-induced antibodies protect you, by neutralizing the pathogen before it can establish an infection.