🧬 Section C · The Victim
P. gingivalis approaching the host... Gingival Sulcus Gingiva free + attached Enamel Dentin Cementum root surface PDL Periodontal Ligament Alveolar Bone
Act 2 Β· The Victim

The Host Periodontium

Four interdependent structures β€” each a target of the villain's attack
Structure 1
Gingiva
Oral epithelium forms the physical barrier. Junctional epithelium seals the sulcus. First contact zone for bacterial challenge β€” and first to show inflammation (gingivitis).
Structure 2
Periodontal Ligament (PDL)
Dense connective tissue fibres anchoring cementum to alveolar bone. Provides shock absorption and proprioception. When destroyed by MMPs β€” tooth mobility follows.
Structure 3
Cementum
Mineralised connective tissue covering the root. PDL fibres insert here (Sharpey's fibres). Resorbs during active bone loss β€” irreversible once lost.
Structure 4
Alveolar Bone
Houses tooth sockets (lamina dura). Remodelled continuously β€” but in periodontitis, osteoclast activity driven by RANKL/IL-17 far exceeds formation. Bone loss is the point of no return.
NF-ΞΊB !
First Line of Defence

The Innate Immune Response

Click each step to activate β€” follow the host defence cascade
STEP 1
PAMPs Released
LPS Β· Fimbriae
Bacterial DNA
β†’
STEP 2
TLR Recognition
TLR2 Β· TLR4
TLR9 on cells
β†’
STEP 3
NF-ΞΊB
NF-ΞΊB Activation
Cytokines released
IL-1Ξ² Β· TNF-Ξ± Β· IL-6
β†’
STEP 4
Neutrophil Recruit
IL-8 chemotaxis
signal to blood
β†’
STEP 5
Phagocytosis
Oxidative burst
bacteria killed
β†’
STEP 6
!
Innate Overwhelmed
Adaptive recruited
T-cells Β· B-cells
PAMPs β€” Pathogen-Associated Molecular Patterns
Conserved molecular structures unique to microorganisms β€” LPS (gram-negative outer membrane), peptidoglycan, bacterial DNA (unmethylated CpG), fimbriae. Recognised by pattern recognition receptors (PRRs) on host innate immune cells.
TLRs β€” Toll-Like Receptors
Transmembrane PRRs on epithelial cells, macrophages and dendritic cells. TLR4 recognises LPS; TLR2 recognises P. gingivalis lipoproteins; TLR9 recognises bacterial DNA. Binding activates NF-ΞΊB β†’ pro-inflammatory gene transcription.
Gingivitis β†’ Periodontitis
Biofilm removed β†’ innate response resolves β†’ gingivitis is reversible. Dysbiosis persists β†’ innate overwhelmed β†’ adaptive arm recruited β†’ irreversible periodontitis.
When Innate Fails

The Adaptive Immune Response

T-cells and B-cells recruited β€” amplifying both defence and destruction

When the innate response cannot clear the dysbiotic biofilm, antigen-presenting cells (macrophages, dendritic cells) process bacterial antigens and activate T-lymphocytes and B-lymphocytes β€” the adaptive arm.

T-Cell Subtypes in Periodontitis
Th1
IFN-Ξ³, IL-2
Macrophage activation
Protective in early disease
Th2
IL-4, IL-5, IL-13
B-cell help
Antibody production
Th17 β˜…
IL-17 Β· IL-22
RANKL ↑ β†’ bone loss
Key driver of destruction
B-Cells β†’ Plasma Cells
Activated B-cells differentiate into plasma cells producing IgG antibodies against periodontal pathogens. While partially protective, the resulting antigen-antibody complexes activate complement β€” amplifying tissue-damaging inflammation.
The Th17 β†’ Bone Loss Pathway β˜…
🦠 Bacterial antigens β†’ Th17 differentiation
↓
πŸ”Ί IL-17 produced β†’ stimulates fibroblasts & osteoblasts
↓
πŸ“ˆ RANKL expression ↑ on stromal cells
↓
πŸ’€ Osteoclast activation β†’ Alveolar bone resorption
Gaffen & Hajishengallis, 2014 Β· Moutsopoulos et al., 2012
Critical Transition
Gingivitis
Innate dominant Β· Reversible Β· No bone loss Β· Neutrophil infiltrate
β†’
Periodontitis
Adaptive dominant Β· Irreversible Β· Bone loss Β· Plasma cell infiltrate
Landmark Model Β· Page & Schroeder, 1976

Histological Disease Progression

Four sequential lesions β€” click each stage to reveal the cellular infiltrate and clinical correlates
Stage 1
Initial Lesion
2–4 days after plaque accumulation
Sulcular Epithelium Vasodilation + exudate
  • Vasodilation of sulcular vessels
  • Neutrophil emigration begins
  • GCF flow increases
  • Collagen loss perivasculary (5–10%)
Clinically: No visible signs β€” subclinical
Stage 2
Early Lesion
4–7 days of plaque accumulation
Sulcular Epithelium T-lymphocyte dominant infiltrate
  • T-lymphocytes dominate (70–80%)
  • Significant collagen loss (60–70%)
  • Junctional epithelium proliferates
  • Fibroblast alteration begins
Clinically: Early gingivitis β€” BOP positive
Stage 3
Established Lesion
Weeks to months
Sulcular Epithelium (proliferating) Plasma cell dominant infiltrate
  • Plasma cells now dominant
  • IgG antibody production
  • Pocket formation begins
  • No bone loss yet (may persist years)
Clinically: Established chronic gingivitis
Stage 4
Advanced Lesion
Established periodontitis
Ulcerated pocket epithelium ALVEOLAR BONE LOSS
  • Alveolar bone loss confirmed
  • Clinical attachment loss
  • Deep periodontal pockets (>4mm)
  • Irreversible β€” only arrestable
Clinically: Periodontitis β€” bone loss on X-ray
Why Some Victims Fall Harder

Host Susceptibility Factors

The same bacterial challenge produces vastly different outcomes β€” the Referee tips the balance
Genetic Polymorphisms
  • IL-1A/B +3954 polymorphism β€” 2Γ— increased severity (Kornman & di Giovine, 1998)
  • TNF-Ξ± promoter variants β€” hyper-inflammatory phenotype
  • FcΞ³RIIIb polymorphism β€” impaired opsonophagocytosis
  • Familial clustering of aggressive periodontitis
Systemic Conditions
  • Diabetes mellitus β€” bidirectional relationship; hyperglycaemia impairs neutrophil chemotaxis and phagocytosis, promotes AGE-mediated tissue damage and pro-inflammatory cytokine overproduction, directly worsening periodontal bone loss
  • HIV/AIDS β€” progressive depletion of CD4+ T-lymphocytes severely compromises immune surveillance; predisposes to necrotising ulcerative gingivitis (NUG) and necrotising ulcerative periodontitis (NUP) β€” rapidly destructive forms of periodontal disease
Behavioural & Environmental
  • Smoking β€” #1 modifiable risk factor. Vasoconstriction masks BOP, impairs neutrophil function, promotes dysbiosis, doubles risk
  • Stress β€” HPA axis activation β†’ cortisol ↑ β†’ immune suppression + crevicular cortisol β†’ bacterial growth
  • Medications β€” phenytoin, ciclosporin, CCBs β†’ gingival enlargement
  • Poor oral hygiene β€” primary initiating factor for all cases
Age & Hormonal Factors
  • Puberty gingivitis β€” oestrogen/progesterone amplify gingival inflammation; Prevotella intermedia uses sex hormones as nutrient
  • Pregnancy gingivitis β€” peaks at 2nd–3rd trimester; progesterone ↑ β†’ immune modulation
  • Menopause β€” oestrogen ↓ β†’ reduced bone density, dry mouth, altered immune response
  • Ageing β€” declining neutrophil efficiency, altered GCF, increased systemic inflammation (inflammageing)
Nutritional Factors
  • Vitamin C deficiency β€” impairs collagen synthesis; classic in scurvy β€” haemorrhagic gingivitis
  • Vitamin D β€” immunomodulatory; deficiency associated with increased periodontal severity
  • Omega-3 fatty acids β€” precursors to resolvins (pro-resolution mediators)
  • Obesity β€” adipokine dysregulation, systemic low-grade inflammation amplifies periodontal disease
Key Concept
"The same villain attacks all hosts β€” but the Referee decides the outcome."
This is why identical plaque scores produce wildly different clinical presentations β€” and why risk factor modification is as critical as biofilm control in periodontal management.
Offenbacher, 1996 Β· Kornman, 2008