Adaptive Immunity – Specificity, Diversity, Memory & Self-Tolerance

Specificity: Recognising “ONE key for ONE lock”

Adaptive (specific) immunity is built on molecular precision.
- Each B- or T-lymphocyte expresses a unique receptor (BCR or TCR)
  • BCR ≈ membrane-bound antibody
  • TCR = heterodimer flavoured for peptide + MHC
- Receptor binds a single epitope (small chemical "face" of an antigen).
Antigen examples & context
- A- and B-blood-group antigens (glycoproteins on RBCs)
- "Self" tissue markers v. foreign markers.
- Transplant medicine is, in practice, a giant “antigen-matching” game.
Epitope = the precise stretch of the antigen recognised.
• One antigen can carry many epitopes → many distinct clones can respond.

Antibody (Immunoglobulin) Architecture

Tetrameric protein: $2$ identical heavy (H) chains + $2$ identical light (L) chains.
- Constant (C) region
  • Same within a class (IgG, IgA, etc.)
  • Signals to other defence molecules (complement) & phagocytes.
- Variable (V) region (VH + VL)
  • Hyper-variable loops contact epitope → exquisite specificity.
Complement & phagocyte recognition
- Complement proteins dock on the heavy-chain C region, punch holes.
- Macrophages/NK cells carry Fc-receptors for the same C region.

Diversity: How can one immune system meet millions of threats?

Somatic gene rearrangement while lymphocytes mature ⇒ huge receptor repertoire.
Statistics: Humans can generate ≈ $10^{9}! -! 10^{11}$ distinct BCR/TCR specificities.
Bottom line: One naïve lymphocyte per epitope exists before infection — hence the “lag” during first sickness.

Clonal Selection, Effector vs Memory

When the rare matching clone meets its epitope:
- Rapid mitosis (clonal expansion).
- Daughter fates:
  • Effector cells – fight now (plasma cells, CTLs).
  • Memory cells – patrol for decades.
Immunological memory
- Secondary response: quicker (hours vs days), bigger (↑ antibody titre), better (higher-affinity IgG, IgA, IgE).
- Explains lifelong or long-term immunity after disease or vaccination.

Self-Tolerance & Autoimmunity

During maturation (bone marrow for B, thymus for T) clones are tested.
- Strong binding to self-antigen ⇒ apoptosis (clonal deletion).
Breakdowns → autoimmune disease.
- Examples
  • Type 1 DM (β-cells destroyed)
  • Graves / Hashimoto thyroid disease
  • Rheumatoid arthritis (joint connective tissue)
  • SLE (systemic lupus erythematosus)
  • Multiple sclerosis (myelin)
- “Escapee” clones may arise after strong infection → molecular mimicry.

B-Lymphocytes & Humoral Immunity

Mission: secrete soluble antibodies into body fluids (humerus = fluid).
Activation requirements
- Antigen cross-links BCR (physical).
- IL-2 from helper T (chemical) → class switching + memory.
Plasma cell (effector B)
- Protein factory: ≈ $2{,}000$ antibody molecules/second.
Without helper T cells (e.g., HIV): weak IgM flare, no class switch, no memory → repeated illnesses.

Antibody Classes & Specialisations

IgM – first made; pentamer; excellent agglutinator; activates complement.
IgG – most abundant in plasma; crosses placenta; opsonises; complement; ADCC with NK cells.
IgA – dimer in secretions (milk, saliva, tears); mucosal shield; passive immunity to infant.
IgD – B-cell receptor co-partner; limited soluble role.
IgE – defends against parasites; when mis-deployed → allergy & anaphylaxis.

What Antibodies Actually Do (All classes can neutralise; some do more)

Neutralisation – "coat & block" virus/bacterial toxin so it cannot dock.
Agglutination – each Ig has ≥2 arms → lattice clumping; easier phagocytosis.
Opsonisation – Fc tail flags “Eat me”; especially IgG.
Complement activation – classical pathway (IgM, IgG) → $C_5b-9$ MAC holes.
Antibody-dependent cellular cytotoxicity (ADCC) – IgG tails engage NK cells.

T-Lymphocytes & Cell-Mediated Immunity

Needed for intracellular foes & rogue self cells (virus-infected, cancer, graft).
Two chief lineages, decided in thymus:
- CD4⁺ Helper T (TH) – recognises peptide + MHC-II; secretes cytokines.
- CD8⁺ Cytotoxic T (CTL) – recognises peptide + MHC-I; kills presenting cell.
Mnemonic: CD8 "ATE" the infected cell; CD4 is "FOR" helping.

Major Histocompatibility Complex (MHC) – “Molecular Billboards”

MHC-I
- On all nucleated cells.
- Displays internal peptides ⇒ "self-report" or "I’m infected".
- Target of CTLs; also crucial in transplant matching.
MHC-II
- Restricted to professional APCs: macrophages, dendritic cells, B cells (and activated T cells).
- Presents exogenous peptides to CD4 helpers.
Organ donation
- Blood transfusions easy (RBCs lack MHC-I).
- Solid organs require close HLA (MHC) match + lifelong immunosuppression.

The Activation Cascade (PUTTING IT ALL TOGETHER)

Phagocyte (often macrophage/dendritic) engulfs pathogen → displays peptide on MHC-II + secretes IL-1.
Naïve CD4⁺ T cell matches epitope + MHC-II (physical) AND senses $IL_1$ (chemical) ⇒ activation & clonal expansion.
Activated Helper T secretes $IL_2$ (and other cytokines).
- B cell that has bound same antigen receives $IL_2$ ⇒ plasma + memory formation, class switch.
- CD8⁺ T that binds peptide + MHC-I on infected cell AND detects $IL_2$ ⇒ becomes CTL.
CTL releases perforin & granzymes → apoptosis; also secretes TNF-α, IFN-γ.

Extracellular vs Intracellular Threats (Division of Labour)

Microbe free in plasma/lymph → Humoral (B-cell) arm predominates.
Virus inside cell, cancer cell, transplanted kidney → Cell-mediated (CTLs & NKs).

Immunological Memory in Practice: Vaccination

Give harmless form of antigen (attenuated, inactivated, sub-unit, mRNA-encoded, etc.).
Body mounts primary response → memory B & T formed.
Later exposure to real pathogen triggers rapid secondary response → no disease.

Passive vs Active Immunity

Active = you make antibodies & memory (infection or vaccine); lasts years-life.
Passive = ready-made antibodies supplied; immediate but temporary.
- Maternal IgG (placenta) & IgA (milk)
- RhoGAM (anti-Rh) to Rh- mothers
- Anti-venom, anti-toxin therapies.

Transplantation & Graft Issues

Rejection = recipient CTLs & antibodies attack graft MHC.
Immunosuppressive drugs (cyclosporine, tacrolimus, steroids) mollify but raise infection risk.
Bone-marrow transplant quirk: graft-versus-host disease (GVHD) – donor immune cells attack recipient.

Hypersensitivity & Allergy

Sensitisation: 1st exposure → TH + B → excess IgE → IgE binds mast-cell Fc.
Re-exposure: allergen cross-links bound IgE → mast-cell degranulation → histamine, leukotrienes.
Clinical spectra
- Local reactions (rhinitis, hives)
- Systemic anaphylaxis
  • Massive vasodilation ↓BP, bronchoconstriction
  • EpiPen (epinephrine) counters: ↑CO, vasoconstriction, bronchodilation.
Anti-histamines block receptor downstream; steroids damp cytokine gene expression.

Food Sensitivity vs Allergy

Lactose intolerance = enzyme deficit (lactase), NOT immune.
True food allergy = IgE mediated (e.g., peanut) → can cause anaphylaxis.

Immunodeficiency States

Congenital: Severe Combined Immuno-Deficiency (SCID) – no B/T cells.
Acquired: HIV infects CD4⁺ T cells via gp120 → progressive helper depletion → opportunistic infections.
Stress hormone cortisol is immunosuppressive → why marathon exams/holidays often precede colds.

Leukaemia & Other Haemato-Oncologic Notes

Malignancy of WBC precursors → overproduction of non-functional leukocytes.
- Crowds marrow → anaemia, thrombocytopenia.
- Immune system useless despite high WBC count.

Ethical / Practical Threads Highlighted

Vaccine hesitancy: risk-benefit & herd immunity considerations.
Immunosuppressant coverage: insurance debates ("Is an expensive kidney-rejection drug still ‘medically necessary’?").
Breastfeeding: passive IgA arguably lowers early-life infection burden.
Gene editing prospects: CRISPR to correct SCID vs germ-line ethics.