Adaptive Immunity: Humoral Immunity

Detailed Notes

11.1 Overview of Adaptive Immunity

The adaptive immune system is the branch of immunity that responds specifically to individual pathogens and improves with each encounter. Unlike the innate system, which responds generically and immediately, adaptive immunity has three hallmark features:

1. Specificity — each lymphocyte (T cell or B cell) carries receptors that recognize a single, unique molecular shape. The adaptive system can distinguish between millions of different foreign molecules.
2. Memory — after the first exposure to a pathogen, the adaptive system retains a population of long-lived cells that mount a faster, stronger response upon re-exposure. This is the basis of vaccination.
3. Self/non-self discrimination — lymphocytes are screened during development, and those that react strongly against the body's own molecules are eliminated. This prevents autoimmunity.

The adaptive response has two arms: humoral immunity (B cells producing antibodies — covered in this topic) and cell-mediated immunity (T cells — Topic 12).

11.2 Antigens and Antigen Presentation

An antigen is any molecule that can be recognized by an adaptive immune receptor (BCR or TCR). Not every antigen triggers a response, however. Immunogenicity — the ability to provoke an immune response — depends on several factors:

• Foreignness: molecules that differ from the body's normal components are more immunogenic.
• Size: larger molecules (typically >10,000 daltons) are better antigens. Small molecules (haptens) can be immunogenic only when attached to a larger carrier protein.
• Chemical complexity: proteins and polysaccharides are strong antigens; lipids and nucleic acids are weak antigens unless complexed with proteins.
• Route and dose: injected antigens are generally more immunogenic than ingested ones.

The specific part of the antigen that a lymphocyte receptor actually binds is called an epitope (or antigenic determinant). A single antigen typically has multiple, different epitopes, each capable of stimulating a distinct lymphocyte clone.

Antigen-Presenting Cells (APCs)

Before T cells can help B cells, antigen must be processed and displayed. Antigen-presenting cells (APCs) ingest foreign material, digest it into peptide fragments, and load those fragments onto MHC molecules for display on the cell surface. The three professional APCs are:

11.3 MHC: Major Histocompatibility Complex

The Major Histocompatibility Complex (MHC) is a set of cell-surface proteins that display peptide fragments for recognition by T cells. In humans, MHC molecules are also called Human Leukocyte Antigens (HLA). There are two structurally distinct classes:

MHC Class I

• Found on: virtually all nucleated cells in the body (not on mature red blood cells, which lack nuclei).
• Presents: endogenous antigens — peptides derived from proteins synthesized inside the cell, including viral proteins if the cell is infected, and tumor antigens if the cell is cancerous.
• Recognized by: CD8+ T cells (cytotoxic T cells). The CD8 co-receptor binds directly to the MHC I molecule, locking the T cell onto the target.
• Outcome: if the peptide is foreign, the CD8+ T cell kills the presenting cell.

MHC Class II

• Found on: professional APCs only — dendritic cells, macrophages, and B cells.
• Presents: exogenous antigens — peptides derived from material the cell has phagocytosed or endocytosed from the extracellular environment.
• Recognized by: CD4+ T cells (helper T cells). The CD4 co-receptor binds MHC II.
• Outcome: the CD4+ T cell becomes activated and secretes cytokines that orchestrate the rest of the immune response — including providing help to B cells.

11.4 B Lymphocyte Development and Maturation

B lymphocytes (B cells) originate and mature in the bone marrow (the "B" stands for bursa-derived in birds, but in humans, bone marrow). Their development proceeds through several stages:

1. Bone marrow origin: hematopoietic stem cells give rise to common lymphoid progenitors, which commit to the B cell lineage.
2. BCR gene rearrangement: each developing B cell randomly rearranges its immunoglobulin gene segments (VDJ recombination), generating a unique B cell receptor (BCR). The BCR is essentially a membrane-bound antibody — an immunoglobulin (Ig) molecule anchored in the plasma membrane. Each B cell displays tens of thousands of identical BCRs on its surface.
3. Tolerance induction — clonal deletion: before leaving the bone marrow, immature B cells are tested against self-antigens. Any B cell whose BCR binds strongly to a self-molecule is eliminated by clonal deletion (apoptosis). This process, called central tolerance, ensures that mature B cells are self-tolerant — they will not attack the body's own tissues. B cells that bind self-antigens weakly may undergo receptor editing (further gene rearrangement to change specificity) rather than deletion.
4. Migration: mature, self-tolerant B cells leave the bone marrow and circulate through the blood and secondary lymphoid organs (lymph nodes, spleen, mucosal lymphoid tissues), where they may encounter their specific antigen.

11.5 Humoral Immune Response: B Cell Activation

Most B cell responses require help from helper T cells — these are called T-dependent antigens (predominantly proteins). The activation process unfolds as a collaborative sequence:

1. Antigen encounter and internalization: the BCR on a naïve B cell binds its specific epitope on the antigen. The B cell internalizes the entire antigen by receptor-mediated endocytosis, processes it in endocytic vesicles, and loads the resulting peptide fragments onto MHC Class II molecules.
2. Antigen presentation to helper T cells: the B cell displays the peptide–MHC II complex on its surface. A CD4+ helper T cell whose TCR recognizes that same peptide–MHC II complex (but typically a different epitope of the same antigen — this is linked recognition) binds to the B cell.
3. Co-stimulation and cytokine help: the T cell–B cell interaction triggers the helper T cell to express CD40 ligand (CD40L), which binds CD40 on the B cell, delivering a crucial co-stimulatory signal. The helper T cell also secretes cytokines, especially:
   • IL-4 — drives B cell proliferation and class switching to IgE
   • IL-5 — promotes B cell differentiation into plasma cells; enhances IgA production
   • IL-6 — stimulates B cell terminal differentiation into antibody-secreting plasma cells
4. Full B cell activation: with both antigen-receptor signaling and T cell help (CD40–CD40L + cytokines), the B cell is now fully activated.

A minority of antigens — large polymeric molecules with repeating epitopes (e.g., bacterial capsular polysaccharides) — can activate B cells without T cell help. These T-independent antigens cross-link many BCRs simultaneously, providing a strong enough signal for activation. However, T-independent responses produce primarily IgM, generate no memory B cells, and lack affinity maturation.

11.6 Clonal Selection and Clonal Expansion

Clonal selection is the central organizing principle of adaptive immunity:

• Among the enormous pool of naïve B cells, each with a different BCR, only the few whose receptors happen to match the invading antigen are "selected" for activation.
• Once activated (with T cell help), that specific B cell undergoes clonal expansion — rapid mitotic division producing thousands of genetically identical daughter cells, all bearing the same BCR.

The clone differentiates into two functional populations:

During clonal expansion, two additional processes refine the antibody response:

• Somatic hypermutation: point mutations are introduced into the variable region genes of the BCR. B cells whose mutated BCR binds antigen more tightly receive stronger survival signals — this is affinity maturation.
• Class switching (isotype switching): the B cell changes the constant region of its antibody from IgM/IgD to IgG, IgA, or IgE while retaining the same antigen specificity. This is directed by cytokines from helper T cells and tailors the effector function to the type of pathogen.

11.7 Primary vs. Secondary Immune Response

The behaviors of the immune system on first and subsequent exposures differ dramatically:

The secondary response is so effective because memory B cells are more numerous than the original naïve B cell precursors and have already undergone affinity maturation and class switching. This is why most pathogens only cause disease once and why booster vaccinations work.

11.8 Antibody (Immunoglobulin) Structure

An antibody, or immunoglobulin (Ig), is a Y-shaped glycoprotein composed of four polypeptide chains:

• Two identical heavy (H) chains — each approximately 440–550 amino acids. The type of heavy chain (μ, γ, α, ε, δ) determines the antibody class.
• Two identical light (L) chains — each approximately 220 amino acids. Light chains are either kappa (κ) or lambda (λ), but any given antibody uses only one type.

The chains are held together by disulfide bonds and non-covalent interactions. The antibody has three key structural and functional regions:

1. Fab region (Fragment, antigen-binding) — the two "arms" of the Y. Each Fab contains:
   • The variable domains of one heavy and one light chain (VH and VL). The tips of the variable domains form the antigen-binding site, whose shape is complementary to the epitope.
   • Hypervariable regions (complementarity-determining regions, CDRs) within the variable domains provide the actual contact surface, making each antibody's binding site unique.
2. Fc region (Fragment, crystallizable) — the "stem" of the Y. Composed of the constant domains of the heavy chains. The Fc region is identical for all antibodies of a given class and mediates effector functions: binding to Fc receptors on phagocytes and NK cells, and activating complement.
3. Hinge region — the flexible segment between Fab and Fc that allows the two Fab arms to move independently, enabling a single antibody to bind two epitopes simultaneously (cross-linking).

11.9 Antibody Classes (Isotypes)

There are five antibody classes, distinguished by their heavy-chain constant region. Each has a distinct distribution and role:

11.10 Antibody-Mediated Effector Functions

Once secreted antibodies bind their target antigen, they do not directly destroy the pathogen. Instead, they "tag" it for destruction by other immune components through several mechanisms:

1. Neutralization: antibodies bind to the surface proteins of viruses or to bacterial toxins, physically blocking their ability to attach to host cell receptors. A neutralized virus cannot enter a host cell; a neutralized toxin cannot damage tissue. This is the only antibody function that works purely through binding — it requires no other cell or protein.

1. Opsonization: the antibody's Fab regions bind the pathogen while the Fc region binds Fc receptors (FcγR) on phagocytes (macrophages, neutrophils). This coats the pathogen in antibodies, making it far easier for phagocytes to recognize, engulf, and destroy it. IgG is the primary opsonizing antibody.

1. Complement activation — classical pathway: when IgM (pentameric) or IgG binds to a pathogen surface, its Fc region can bind and activate the C1 complex (C1q, C1r, C1s), the first component of the complement cascade. This triggers the classical pathway, leading to:
   • Formation of the membrane attack complex (MAC) that lyses the pathogen
   • Generation of C3b (opsonin) that further enhances phagocytosis
   • Release of C3a and C5a (anaphylatoxins) that promote inflammation and recruit phagocytes

1. Agglutination and precipitation: antibodies can cross-link multiple antigen-bearing particles (e.g., bacteria, red blood cells) into clumps — agglutination — or cross-link soluble antigens into insoluble complexes — precipitation. These clumps are more easily filtered by the spleen and phagocytosed.

1. Antibody-dependent cell-mediated cytotoxicity (ADCC): antibodies (especially IgG) coating a target cell are recognized by NK (natural killer) cells via their Fc receptor (FcγRIII / CD16). The NK cell releases cytotoxic granules (perforin and granzymes) that kill the antibody-coated target. ADCC is important for eliminating virus-infected cells and tumor cells.

11.11 Clinical Correlations

Monoclonal Antibodies

Monoclonal antibodies (mAbs) are laboratory-produced antibodies with a single, defined specificity — all derived from a single B cell clone. They are used therapeutically to target specific molecules on cancer cells (e.g., rituximab targets CD20 on B cell lymphomas), block inflammatory cytokines (e.g., adalimumab targets TNF-α in rheumatoid arthritis), or deliver conjugated toxins/radioisotopes to tumors. Nomenclature: drug names ending in "-mab" (monoclonal antibody).

Multiple Myeloma

Multiple myeloma is a cancer of plasma cells. A single plasma cell clone proliferates uncontrollably in the bone marrow, producing enormous quantities of a single, non-functional monoclonal antibody (M-protein) while crowding out normal hematopoietic cells. Patients present with bone pain (lytic lesions), hypercalcemia, renal failure, anemia, and recurrent infections (because functional antibody diversity is suppressed). Diagnosis involves serum protein electrophoresis showing an M-spike and Bence Jones proteins (free light chains) in the urine.

Agammaglobulinemia

Agammaglobulinemia (specifically X-linked agammaglobulinemia, or Bruton's disease) is a primary immunodeficiency caused by a mutation in the Bruton's tyrosine kinase (BTK) gene, which is required for B cell maturation. Affected individuals have virtually no B cells, no plasma cells, and extremely low levels of all immunoglobulin classes. They suffer from recurrent pyogenic bacterial infections beginning around 6 months of age (when maternal IgG wanes). Treatment is intravenous immunoglobulin (IVIG) replacement.