Principles of innate and adaptive immune responses

The two arms

The innate immune system (rapid, non-specific - physical barriers, phagocytes, complement) and adaptive immune system (slower, highly specific, generates immunological memory - B and T lymphocytes) work together to recognise and eliminate pathogens, with the innate system also activating and shaping the subsequent adaptive response
Foundational immunology underlying essentially all of infectious diseases practice, from understanding why certain patients (asplenic, complement-deficient) are at elevated risk of specific infections to how vaccines work
Innate immunity responds within minutes-hours via pattern recognition receptors detecting conserved microbial structures, activating phagocytosis, complement, and inflammatory cytokine release - fast but lacking memory or high specificity
Adaptive immunity (B cells producing pathogen-specific antibodies; T cells providing direct cytotoxic killing of infected cells and helper function coordinating the broader response) takes days to develop on first exposure but generates lasting immunological memory, enabling a much faster, stronger response on re-exposure - the basis of vaccine-induced protection

Specific immune deficits predict specific infection susceptibility patterns - asplenia/hyposplenism increases risk of encapsulated organism infection (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis) given the spleen's role in opsonised bacteria clearance; complement deficiency particularly increases Neisseria infection risk; T-cell deficiency increases risk of intracellular pathogens (viruses, fungi, mycobacteria, certain parasites)

Recognising a patient's specific immune deficit pattern (asplenic, complement-deficient, T-cell deficient) directs targeted prophylactic vaccination and, in some cases, prophylactic antibiotics, matched to that deficit's characteristic infection risk
Vaccine design exploits adaptive immune memory generation, explaining why booster doses/schedules are needed to achieve durable protection for many vaccines

Asplenia and encapsulated organism risk (pneumococcal, meningococcal, Hib vaccination priority), complement deficiency and recurrent Neisseria infection, T-cell deficiency and opportunistic infection susceptibility
Asplenic and hyposplenic patients face a specific, elevated risk of overwhelming infection with encapsulated organisms (pneumococcus, meningococcus, Haemophilus influenzae) given the spleen's specific role in clearing opsonised bacteria - this drives a distinct, more intensive vaccination and sometimes prophylactic antibiotic strategy in this population

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