Immune responses, adaptive and innate

The two arms

The immune system comprises two interacting arms: innate immunity (immediate, non-specific, no memory) and adaptive immunity (delayed onset, antigen-specific, generates immunological memory)
Innate responses activate within minutes-hours; adaptive responses take days to develop on first exposure but respond within hours on re-exposure (immunological memory)

Innate: physical barriers (skin, mucosa), phagocytes (neutrophils, macrophages), NK cells, complement, pattern recognition receptors (e.g. Toll-like receptors) recognising conserved pathogen structures (PAMPs)
Adaptive: T cells (cell-mediated - CD4 helper, CD8 cytotoxic, Tregs) and B cells (humoral - differentiate into plasma cells producing antibody) - recognise highly specific antigens via randomly generated receptors (V(D)J recombination), with clonal selection and expansion of the matching clone
The two arms are interdependent - dendritic cells (innate) present antigen to prime the adaptive response; antibody (adaptive) opsonises pathogens for more efficient innate phagocytosis

Immunodeficiency patterns localise to the affected arm: recurrent bacterial/fungal infection with poor wound healing suggests innate/phagocyte defects; recurrent viral/opportunistic infection suggests T cell defects; recurrent encapsulated bacterial infection (sinopulmonary) suggests antibody/B cell defects

Vaccination exploits adaptive immune memory - deliberately triggering a primary response so a subsequent true exposure triggers a faster, larger secondary (memory) response

Primary immunodeficiency syndromes (localise by which arm is affected), vaccination principles, autoimmunity (adaptive immune system attacking self)

The clonal selection and memory generated by a single antigen exposure (natural infection or vaccination) can persist for years to decades, the physiological basis for durable vaccine protection

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