1.5 Host Immunity, Defenses & Susceptibility Factors

1.5 Host Immunity, Defenses & Susceptibility Factors

The development of infectious disease represents a dynamic equilibrium between microbial virulence and host defense mechanisms. Host defenses are structured into non-specific innate immune barriers and highly specialized adaptive immune responses. Infection preventionists (IPs) must evaluate patient susceptibility factors, immune deficiencies, and medical interventions to identify individuals at elevated risk for opportunistic healthcare-associated infections.

1. Architecture of Host Immunity: Innate vs. Adaptive

Host immunity operates through two complementary arms that coordinate cellular and humoral responses to eliminate invading pathogens.

Innate (Non-Specific) Immunity

Innate immunity provides immediate, non-specific protection without requiring prior pathogen exposure, and lacks immunological memory.

• Physical & Anatomical Barriers: Intact skin (stratum corneum with desquamation), mucous membranes, mucociliary escalator of the respiratory tract, and tight junctional networks between epithelial cells.
• Physiological & Chemical Barriers: Acidic pH of the stomach (pH < 2.0) and vaginal tract, lysozyme in tears and saliva, secretory phospholipase A2, and antimicrobial peptides (defensins, cathelicidins).
• Cellular Responders: Polymorphonuclear neutrophils (PMNs) perform rapid phagocytosis; macrophages engulf debris and present antigens; Natural Killer (NK) cells identify and lyse virus-infected cells and tumor cells lacking MHC Class I molecules; dendritic cells act as primary antigen-presenting cells (APCs).
• The Complement System: A cascade of plasma proteins activated via three pathways:
  1. Classical Pathway: Triggered by antigen-antibody (IgM or IgG) complexes.
  2. Lectin Pathway: Triggered by mannose-binding lectin binding to microbial surface carbohydrates.
  3. Alternative Pathway: Triggered by direct complement component C3 hydrolysis on microbial surface molecules.
  • Outcome: Complement activation leads to C3b opsonization (enhancing phagocytosis), C3a/C5a anaphylatoxin release (recruiting inflammatory cells), and assembly of the Membrane Attack Complex (MAC, C5b–C9), which drills lethal pores into Gram-negative bacterial cell walls.

2. Humoral vs. Cell-Mediated Adaptive Immunity

Adaptive immunity develops over days to weeks following antigen recognition, exhibits exquisite antigen specificity, and generates long-lasting immunological memory.

Immunoglobulin Classes & Functional Roles

• IgM: Pentameric structure; first antibody isotype produced during primary immune response; potent complement activator.
• IgG: Most abundant serum immunoglobulin (~80%); long half-life (21 days); crosses the placenta to confer passive neonatal immunity; mediates opsonization, complement activation, and toxin neutralization.
• IgA: Predominants in mucosal secretions (saliva, tears, colostrum, GI and respiratory tracts) as a dimer with a secretory component; blocks microbial attachment to mucosal surfaces.
• IgE: Binds to high-affinity Fc-epsilon-RI receptors on mast cells and basophils; mediates Type I hypersensitivity reactions and defense against parasitic helminths.
• IgD: Functions primarily as an antigen receptor on naive B cell membranes.

T-Lymphocyte Subsets

• CD4+ Helper T Cells (Th): Recognize antigens presented on Major Histocompatibility Complex (MHC) Class II molecules on APCs. Th1 cells produce IFN-gamma to activate macrophages against intracellular pathogens; Th2 cells produce IL-4, IL-5, and IL-13 to stimulate B-cell antibody production.
• CD8+ Cytotoxic T Lymphocytes (CTLs): Recognize endogenous peptides presented on MHC Class I molecules expressed on all nucleated host cells. CTLs destroy virus-infected or malignant cells by releasing perforin (pore-forming protein) and granzymes (serine proteases triggering apoptosis).

3. Host Susceptibility Factors & Defense Breakdown

Pathogen establishment occurs when host defenses are compromised by underlying medical conditions, therapeutic interventions, or physical disruptions.

Neutropenia & Phagocytic Defects

Neutrophils represent the primary defense against bacterial and fungal invaders.

• Profound Neutropenia: Defined as an Absolute Neutrophil Count (ANC) <500 cells/uL (or predicted to drop <500 cells/uL over 48 hours). Seen secondary to cytotoxic chemotherapy, aplastic anemia, or bone marrow transplantation.
• Clinical Implication: Neutropenic patients cannot mount a typical inflammatory response (pus formation, localized swelling). Fever (>= 38.3°C) is often the sole indicator of severe infection. Neutropenic fever is a medical emergency requiring immediate empirical broad-spectrum antipseudomonal beta-lactam therapy (e.g., cefepime, piperacillin-tazobactam, or meropenem).

Immunosuppressive Therapies & Corticosteroids

• Corticosteroids: High-dose or prolonged glucocorticoid therapy inhibits cytokine transcription, induces T-cell apoptosis, suppresses macrophage antigen presentation, and dampens fever response, predisposing patients to opportunistic bacterial, fungal, and viral reactivation (e.g., Herpes Zoster, Cytomegalovirus).
• Biologic Agents: Tumor Necrosis Factor-alpha (TNF-alpha) inhibitors (e.g., infliximab, adalimumab) disrupt granuloma formation, dramatically increasing the risk of reactivation of latent Mycobacterium tuberculosis and endemic dimorphic fungi (Histoplasma, Coccidioides).

Invasive Medical Devices & Anatomical Breakdown

• Skin & Mucosal Disruption: Severe burn injuries, surgical wounds, pressure injuries, and chemotherapy-induced mucositis breach primary anatomical barriers, allowing skin and mucosal commensals (S. aureus, P. aeruginosa, Candida) direct tissue entry.
• Indwelling Invasive Devices: Vascular catheters, urinary catheters, endotracheal tubes, and prosthetic implants bypass physical host barriers, provide synthetic substrates for biofilm accretion, and impair local mucosal clearance mechanisms.

Extremes of Age

• Neonates: Immature immune system characterized by reduced complement levels, impaired neutrophil chemotaxis, and low endogenous immunoglobulin synthesis (relying on transplacental maternal IgG).
• Elderly (Immunosenescence): Age-related immune decline marked by thymic involution, decreased T-cell receptor diversity, blunted antibody response to vaccination, and chronic low-grade systemic inflammation ("inflammaging"). Elderly individuals frequently present with atypical infection signs (e.g., acute confusion, functional decline, hypothermia instead of fever).

4. Opportunistic Pathogens in Immunocompromised Hosts

Opportunistic pathogens rarely cause severe disease in immunocompetent individuals but produce life-threatening infections in hosts with impaired immune surveillance.