Chapter 17 Notes – Adaptive Immunity: Humoral Response & Antibodies

Adaptive (Acquired / Specific) Immunity

• Adaptive immunity = acquired/specific immunity

  • Develops with time & exposure to germs (natural infection or vaccination).

  • Provides flexible, highly targeted defense.

  • Features a primary response (first exposure) followed by a faster, stronger secondary/memory response upon re-exposure.

Two Major Arms of the Adaptive Immune System

• Humoral (antibody-mediated)

  • Key cell: B lymphocyte (B cell)

  • Effector: antibodies (immunoglobulins, Ig)

  • Targets extracellular pathogens, toxins, antigens found in body “humors” (fluids).

• Cell-mediated (cellular) immunity

  • Key cell: T lymphocyte (T cell), matured in the thymus.

  • Effector molecules: cytokines secreted by activated T cells.

  • Attacks infected cells, fungi, parasites, and transformed (pre-cancer) cells.

Detailed Focus on Humoral Immunity (today’s emphasis)

Antigens (Ag)

• Definition: Substances—usually components of invading microbes or their toxins—that elicit antibody production.

• Strongest antigens: polypeptides & complex carbohydrates (provide shape, charge, complexity).

• Poor antigens: nucleic acids (repetitive sugar-phosphate backbone) & lipids (long repetitive C–H chains).

• Antigen entry into body

  • Natural infection (penetration, colonization)

  • Vaccination (whole germ or fragment administered before exposure)

Epitopes & Haptens

• Epitope / Antigenic determinant: specific portion of antigen bound by an antibody (antibody rarely contacts entire antigen).

• Hapten: molecule too small to evoke immune response until attached to a larger carrier (e.g., binds serum albumin, then becomes immunogenic).

B-cell Selection & Activation

• Each naïve B cell displays unique B-cell receptors (BCRs) (membrane-bound antibodies).

• Binding of matching antigen + helper T-cell assistance ⇒ B cell differentiates into a plasma cell (antibody-secreting factory).

Antibodies (Immunoglobulins, Ig)

• Large, globular proteins produced by plasma cells.

• Millions of distinct antibodies circulate/tissue-bathe the body at any moment.

• Valence = number of antigen-binding sites per antibody molecule.

  • Standard Ig monomer valence: $2$.

Fine Structure of a Typical Antibody

• Composition: 2 identical light chains + 2 identical heavy chains.

  • Chains joined by disulfide bridges (covalent S–S bonds via cysteine residues).

• Regions

  • Variable (V) region: hypervariable tips of both heavy & light chains; forms antigen-binding site (shape/charge complementarity for epitope).

  • Constant (C) region / crystallizable fragment (Fc): stem + lower portion of heavy chains; mediates effector functions (binds complement, macrophage Fc receptors, etc.).

• Symmetry:

  • Two light-chain sequences are identical; two heavy chains identical ⇒ both binding sites match.

• Flexibility: hinge region allows Y-shape to splay or be T-shaped.

Five Major Antibody Isotypes ("G MADE")

IgM – “Monster / Massive / First Responder”

• Structure: pentamer (5 Ig units) held by joining (J) chain.

  • Total potential binding sites: $5 \text{ monomers} \times 2 = 10$ (all identical for same epitope).

• Serum proportion: $\approx 6\%$.

• Half-life: $\sim 5\text{ days}$ (short).

• Produced first after antigen exposure; especially effective against Staphylococcus & Streptococcus.

• Activates classical complement pathway.

• Also exists as monomeric BCR on naïve B cells.

IgG – “Good All-Around”

• Structure: monomer.

• Most abundant serum antibody: highest percentage (dominant class).

• Distribution: blood, lymph, intestine.

• Persistence: almost one month.

• Effector abilities:

  • Activates classical complement.

  • Crosses placenta ⇒ passes maternal immunity to fetus.

• Typical response timeline: follows IgM (“class switch”).

IgA – “Secretory Surface Shield”

• Structure: dimer (two monomers + secretory component) in secretions; monomer in serum.

• Production quantity: roughly double IgM output.

• Localization: tears, saliva, mucus, mucosal linings, breast milk.

• Functions:

  • Neutralizes pathogens/toxins before entry (acts like bouncer outside the “bar”).

  • Provides passive immunity to neonate via mother’s milk (post-birth protection).

• Does not cross placenta; does not activate complement.

IgD – “Developmental Marker”

• Primarily membrane-bound on B cell surface as BCR in early activation stages.

• Low serum concentration; specific functions less well defined.

IgE – “Eosinophils, Allergy, Worms”

• Fc region binds mast cells & basophils.

• Triggers histamine release → inflammatory / allergic responses.

• Important in defense against parasitic worms (helminths).

Comparative Highlights & Connections

• Timeline of response

  • First: $\text{IgM} \rightarrow \text{class switch} \rightarrow \text{IgG / IgA}$ depending on location.

• Maternal immunity

  • In utero: $\text{IgG}_{\text{mother}} \to \text{fetus}$ via placenta.

  • After birth: $\text{IgA}_{\text{mother}} \to \text{infant}$ via breast milk.

• Complement activation: IgM & IgG (classical pathway) lead to:

  • Opsonization

  • Membrane attack complex (MAC)

  • Inflammation enhancement

• Immune surveillance vs cancer: Cell-mediated arm (T cells) can detect & remove transforming cells; will be explored later.

Ethical, Philosophical & Practical Implications

• Vaccination exploits memory/secondary response for rapid future protection.

• Maternal transfer of antibodies underscores evolutionary strategies for neonatal protection before self-immunity matures.

• Surface IgA emphasizes the concept of preventive immunity at barriers (keep threats from ever entering systemic circulation).

Looking Ahead

• Next lecture: B-cell selection, clonal expansion, plasma-cell differentiation & detailed antibody synthesis.

• Will revisit cell-mediated immunity once humoral mechanisms are fully understood.