How the Human Body Fights Infection: A Comprehensive Defense Overview

The immune system employs layered defenses: physical barriers, innate immunity, adaptive immunity, and memory.

1. Physical & Chemical Barriers

• Skin (keratin, acidic pH, microbiome)
• Mucous membranes (ciliary clearance, mucus trapping)
• Secretions: lysozyme (tears, saliva), gastric acid, defensins

2. Innate Immunity (Hours–Days)

• Phagocytes: Neutrophils (acute), macrophages (tissue-resident), monocytes
• Natural Killer (NK) cells: Kill virus-infected/tumor cells
• Inflammation: Vasodilation, chemokines, complement activation
• Antimicrobial peptides, interferons (antiviral)

3. Adaptive Immunity (Days–Weeks)

• Humoral (B cells): Plasma cells → antibodies (neutralize, opsonize, activate complement).
• Cell-mediated (T cells):
  • CD4+ Th1/Th2/Th17/Tfh: Orchestrate responses
  • CD8+ cytotoxic T lymphocytes (CTLs): Kill infected cells
  • Tregs: Suppress excess
• Germinal centers: Affinity maturation, class switching, memory B cells.

4. Complement System

Cascade enhancing opsonization (C3b), inflammation (C3a, C5a), lysis (MAC).

5. Fever

Pyrogens (IL-1, TNF) → hypothalamic set-point ↑ → inhibits pathogens, enhances immunity.

6. Microbiome

Colonization resistance: Commensals compete for niches, produce bacteriocins, train immunity.

7. Immunological Memory

Memory B/T cells → faster, stronger secondary response. Basis of vaccination.

8. Lymphatic System

Drains interstitial fluid, transports antigen/APCs to lymph nodes for immune activation.

Factors Affecting Immunity

• Age (immunosenescence), nutrition, sleep, stress, chronic disease, medications.