Body's Defense Mechanisms Against Foreign Bodies: A Comprehensive Overview

The immune response to foreign bodies (pathogens, toxins, allergens) is a layered, coordinated process involving innate and adaptive arms.

1. Physical & Chemical Barriers (First Line)

Skin: Keratinized epithelium, acidic pH, antimicrobial peptides, microbiome competition.
Mucous membranes: Mucus traps particles; cilia clear them; lysozyme, defensins, IgA in secretions.
Physiologic clearance: Cough, sneeze, peristalsis, urine flow, tears.

Phagocytes: Neutrophils (acute), macrophages (tissue-resident), monocytes. Engulf → phagolysosome → kill (ROS, enzymes, NO).
Natural Killer (NK) cells: Kill virus-infected and tumor cells (missing self-MHC).
Complement system: Classical, lectin, alternative pathways → opsonization (C3b), chemotaxis (C5a), MAC lysis (C5b-9).
Inflammation: Vasodilation, increased permeability, leukocyte recruitment (selectins, integrins, chemokines). Cardinal signs: rubor, tumor, calor, dolor, functio laesa.
Pattern recognition receptors (PRRs): TLRs, NLRs, RLRs detect PAMPs/DAMPs → cytokine/IFN production.

Antigen presentation: DCs/macrophages/B cells present peptide-MHC to T cells.
CD4+ T helper subsets: Th1 (intracellular), Th2 (helminths/allergy), Th17 (extracellular bacteria/fungi), Tfh (B cell help), Treg (suppression).
CD8+ Cytotoxic T cells: Kill infected/abnormal cells (MHC I + peptide).
B cells → Plasma cells: Produce high-affinity antibodies (IgM → IgG/IgA/IgE) → neutralization, opsonization, ADCC, complement activation.
Germinal centers: Affinity maturation, class switching, memory B/T cell generation.

Tregs, IL-10, TGF-β, PD-1/CTLA-4: Prevent autoimmunity, limit collateral damage.
Memory cells: Rapid, robust secondary response (anamnesis). Basis of vaccination.

Antigenic variation, latency, immunosuppression, molecular mimicry, complement inhibition—driving co-evolution.

This layered defense allows coexistence with a microbial world while maintaining integrity.