Specific Host Defenses – Detailed Study Notes

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Topic: Specific Host Defenses – Immunology & Immunity

• This section introduces the overall subject of how the body defends itself specifically (as opposed to non-specific/innate defenses).

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Two Major Types of Immunity

1. Innate (Genetic / Inborn) Immunity

   • Genetically inherited, present in every member of a species.

   • Non-specific: same response to many pathogens.

2. Adaptive (Acquired) Immunity – Specific, requires exposure.
   a. Active (host produces its own antibodies)
   b. Passive (antibodies made outside host)
   • Naturally acquired adaptive – via disease recovery or trans-placental transfer.
   • Artificially acquired adaptive – via injection (vaccines, toxoids, immune sera).

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Flow-Chart of Immunity Types (Figure 17-1)

• INNATE ↔ ACQUIRED
  • Acquired splits into ACTIVE (own Abs) and PASSIVE (ready-made Abs).
  • Both Active & Passive subdivide into NATURAL vs ARTIFICIAL.

• Natural Active = exposure to agent; Natural Passive = maternal Abs.

• Artificial Active = immunization; Artificial Passive = antibodies from other sources.

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Detailed Definitions

1. Active Immunity

   • Natural active: post-infection antibody production.

   • Artificial active: vaccination with live, attenuated, dead organisms, or toxoids.

2. Passive Immunity

   • Host does NOT produce Abs.

   • Natural passive: maternal IgG across placenta or IgA in colostrum.

   • Artificial passive: injection of exogenous Abs (e.g., snake antivenom from horse/rabbit).

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Fundamental Characteristics of the Immune System

• Antigen (Ag / Immunogen): Any foreign compound that can elicit immune response; typically proteins or polysaccharides located on microbial surfaces, transplanted tissues, foods, etc.

• Large proteins carry multiple epitopes (antigenic determinants) – individual sites to which specific Ab molecules bind.

• Antigens may be found on viruses, bacteria, or even human cells (important for autoimmunity & transplant rejection).

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Antigen & Epitope Examples (Figures 17-2a, 17-2b)

• Viral capsids/spikes exhibit distinct epitopes; antibodies can target flagellar, pilus, capsule, or cell-wall epitopes of bacteria.

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Hapten Concept

• Hapten: Small molecule (low MW) that is non-immunogenic unless bound to a large carrier protein.
  • Once conjugated, the hapten functions as an epitope.
  • Example: Penicillin binds serum proteins ⇨ becomes antigenic ⇒ allergic reaction.

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Antibody (Ab / Immunoglobulin, Ig)

• Protein produced by B cells in response to specific Ag; each Ab binds a single unique epitope.

• Specificity: ONE Ab type neutralizes ONE Ag.

• Memory blueprint stored in Memory T cells (note: modern texts place memory in B & T cells).

• Titer: Quantitative measure of Ab or Ag needed for a set reaction.

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Cells & Tissues of the Immune System

• All lymphocytes derive from pluripotent stem cells (originate in yolk sac → migrate via umbilical circulation).

1. B Lymphocyte (B cell) Development

   • Differentiation occurs in bone marrow (analogous bursal tissue in birds).

   • Mature B cells populate lymph nodes, spleen, tonsils, adenoids, GALT, appendix, Peyer’s patches.

2. T Lymphocyte (T cell) Development

   • Stem cells migrate to thymus (esp. in childhood) for differentiation.

   • Adult thymus still functions but less active.

• Figure 17-3: Stem-cell migration sequence (yolk sac → fetal liver → bone marrow).

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Lineage Chart

• Pluripotent stem cell → Lymphoid vs Myeloid lines.

• Lymphoid → Naïve T cell → Helper & Cytotoxic T (after thymic maturation) and B cell → Plasma cell (after Ag activation).

• Plasma cells secrete antibodies.

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• Comparative anatomy figure (bursa of Fabricius vs mammalian organs).

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T-Cell Subsets

• Cytotoxic (Tc / killer)

• Delayed-hypersensitivity (TDH)

• Helper (TH)

• Regulatory (Treg / suppressor)

• Natural Killer (NK) cells – large granular lymphocytes lacking specific B/T markers; kill virus-infected or tumor cells non-specifically.

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Interactive Roles (Figure)

• Helper T cytokines activate B cells and cytotoxic T cells.

• Cytotoxic T directly kill Ag-bearing cells.

• Ab and cytokines guide phagocytes, complement, NK cells.

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Dual Nature of Immunity

1. Humoral Immunity – B cells + circulating Abs; effective against extracellular toxins, bacteria, viruses BEFORE cell entry.

2. Cell-Mediated Immunity (CMI) – T cells; targets intracellular pathogens, fungi, parasites, cancer cells, transplanted tissue.

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General Properties of Immune Response

1. Self vs Non-self Recognition

2. Specificity

3. Heterogeneity (Diversity)

4. Memory

• Clonal Selection Hypothesis (Burnet, 1950s): Each lymphocyte bears unique receptor; encounter with its cognate Ag triggers clonal expansion.

• Clonal Deletion: Self-reactive B or T cells destroyed during development; basis of tolerance.

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Clonal Selection & Deletion (Figures 17-5, 17-6)
Sequence:
Stem-cell → pre-B → Ag exposure → TH collaboration → B-cell clone ⇨ Plasma cells (Ab secretion) + Memory cells.

• Clonal deletion ensures removal of self-reactive clones (illustrated by binding self Ag → apoptosis).

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Tolerance & Clinical Manipulation

• Induced tolerance via irradiation (cancer therapy) or immunosuppressants (transplant).

• Drawback: susceptibility to infections.

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Specificity, Diversity, Memory in Detail

• Full specificity by age 2–3 yrs.

• Cross-reaction: Ab binds structurally similar epitopes (e.g., Treponema pallidum hapten resembles human cardiac tissue).

• Immune diversity enables recognition of vast epitope range.

• Memory permits faster, stronger secondary responses.

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Humoral Immunity Mechanics

• B cell must:

  1. Recognize specific Ag (surface Ig acts as receptor).

  2. Initiate protective responses (Ab production).

• Steps: Ag binds B-cell receptors ⇨ Activation ⇨ Clonal expansion into Plasma & Memory cells.

• Ag fragments displayed with MHC II; TH cells recognize Ag-MHC II and secrete IL-2 & other cytokines → drive B-cell proliferation.

• Memory B cells remain dormant until re-exposed to same Ag.

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(Figure shows TH interaction with macrophage & B cell; highlights Class II MHC involvement, receptor engagement, nuclear activation.)

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Antibody Structure

• Basic unit: Y-shaped tetramer; $2H+2L$ chains linked by disulfide bonds.
  • Fab (fragment antigen binding): variable regions (VH & VL) – specificity.
  • Fc (fragment crystallizable): constant region of heavy chains – determines class, binds complement & macrophage Fc receptors, crosses placenta (for IgG).

• Complement-binding site on Fc; carbohydrate side chains present.

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Five Immunoglobulin Isotypes

• Heavy chain determines class.

1. IgG – $\gamma$ chain

2. IgA – $\alpha$ (secretory component + J-chain in dimer)

3. IgM – $\mu$ (pentamer with J-chain)

4. IgD – $\delta$

5. IgE – $\varepsilon$

• Functional Highlights
  • Fab: epitope binding.
  • Fc: opsonization & complement activation.
  • Opsonization: Ab coating enhances phagocytosis.

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Detailed Class Characteristics (Table 17.1)
IgG
• Monomer, 80% serum, crosses placenta, half-life $\approx 23\,\text{d}$.
• Functions: complement fixation, opsonization, toxin/virus neutralization, fetal protection.
IgM
• Pentamer, 5–10% serum, first Ab made, half-life 5 d, strong agglutination.
IgA
• Dimer in secretions (tears, saliva, mucus, milk); 10–15% serum; mucosal protection; half-life 6 d.
IgD
• Monomer, membrane-bound on B cells; initiates immune response; half-life 3 d.
IgE
• Monomer, binds mast & basophils; allergic response and helminth lysis; half-life 2 d.

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Primary vs Secondary Response

• Primary: Lag phase; IgM appears first, followed by IgG; peak titer after 1–10 weeks.

• Secondary: Memory cells → rapid clonal expansion; IgG rises quickly & higher titer; IgM response similar or slightly faster.

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Ag–Ab Reaction Types & Functions

1. Neutralization: IgA/IgG block adhesion of pathogens/toxins to host tissues.

2. Opsonization: IgG or IgM coat microbes, facilitating phagocytosis.

3. Agglutination: IgM (high valency) cross-links microbes, reducing infectious units.

4. Complement Activation: IgG/IgM trigger classical pathway → $\text{C}1$ binding → inflammation, membrane attack complex (MAC) $\rightarrow$ lysis.

5. Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC): Ab tags large targets (parasites); eosinophils & NK cells bind Fc region and kill extracellularly.

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B-Cell Activation Paths

• T-Independent Ag: Directly stimulate B cells → mainly IgM, no memory.

• T-Dependent Ag: Require TH collaboration (cytokines) → isotope switching (IgG, IgA, etc.), memory formation.

• Figure illustrates MHC II presentation, TH activation, proliferation into Plasma & Memory cells.

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Escape & Cooperative Defense

• Microbes evading mucosal IgA encounter lymphoid tissues; macrophages process Ag; helper T assist B cells.

• Outcome depends on toxin vs whole cell:
  • Toxin → neutralization (IgG) + antibiotics to kill bacteria.
  • Bacterial cells → opsonization & agglutination (IgM/IgG).
  • Intracellular agents → complement-mediated lysis (IgG/IgM).
  • Parasites → ADCC with eosinophils/NK.

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Visual Summary of Effector Mechanisms

• Diagrams for neutralization, opsonization, agglutination, complement lysis.

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Comprehensive Humoral Immunity Flow-Chart

1. Stem cells → B cell repertoire.

2. Clonal deletion removes self-reactive clones.

3. Macro-presented Ag + TH → clonal selection.

4. Sensitized B cells → Plasma + Memory.

5. Plasma secretes Abs → Neutralization / Opsonization / Cell lysis via complement.

Key Equations & Terms

• Complement fixation start: $\text{Ab} + \text{Ag} + \text{C}1 \rightarrow \text{C}1\text{–Ab–Ag}$ complex.

• Titer definition (conceptual, not formula): the reciprocal of highest dilution giving positive test.

Integrative Notes & Implications

• Ethical/Clinical: Immunosuppression (transplants, cancer therapy) lowers resistance to infection.

• Vaccination: Artificial active immunity builds memory without disease; herd immunity arises when enough individuals mount memory responses.

• Autoimmunity: Failure of clonal deletion/tolerance can produce cross-reactions (e.g., rheumatic fever from streptococcal M protein resemblance).

• Allergy (IgE): Hapten/carrier conjugates (e.g., penicillin) illustrate drug allergies.

• Serotherapy: Antivenoms exemplify artificial passive immunity—rapid but temporary (no memory, serum sickness possible).

Linked Foundational Principles

• MHC Restriction: T-cell recognition requires Ag presented with self-MHC.

• Somatic Recombination (VDJ): Underlies diversity of Ig and TCR (not explicit in transcript but critical background).

• Complement Paths: Classical (Ab-dependent) vs Alternative/Lectin (Innate); IgG/IgM only Abs that efficiently fix complement.

Cross-Lecture/Real-World Connections

• GALT concept connects to microbiome lectures (intestinal immunity).

• NK cells link to oncology/immunotherapy (e.g., CAR-NK).

• Passive maternal IgG underscores importance of timing infant vaccinations (maternal Ab can neutralize live vaccines).

Recap: Study Checklist

✔️ Know definitions: antigen, epitope, hapten, antibody, titer.
✔️ Memorize five Ig classes – structure, location, half-life, function.
✔️ Diagram clonal selection/deletion and primary vs secondary response graphs.
✔️ Differentiate humoral vs cell-mediated immunity tasks.
✔️ Recall effector mechanisms (neutralization, opsonization, complement, ADCC).
✔️ Apply passive vs active, natural vs artificial categories to examples.