Synopsis
Connect immune recognition, signalling and effector pathways to infection control, hypersensitivity, autoimmunity, immunodeficiency, vaccination and the interpretation of common immune investigations.
- Innate immunity responds rapidly through barriers, pattern-recognition receptors, phagocytes, natural killer cells, cytokines and complement; it also initiates adaptive responses.
- Adaptive immunity uses clonally distributed antigen receptors: B cells generate antibody responses and T cells coordinate or kill through peptide–MHC recognition.
- Antigen presentation on MHC I primarily engages CD8 T cells, while MHC II on professional antigen-presenting cells primarily engages CD4 T cells.
Reasoning priorities
Quantify major circulating leukocyte populations and identify cytopenia or eosinophilia.
Use age-specific ranges and trends; normal counts do not prove normal cell function, and infection or medicines can cause secondary changes.
Worked reasoning
Inputs: a young adult has two episodes of invasive meningococcal disease, normal neutrophil count, normal quantitative immunoglobulins and no history of opportunistic infection.
- Map the organism and phenotype: repeated invasive Neisseria infection is a focused clue to impaired terminal complement-mediated killing rather than a general infection frequency label.
- Use existing normal data to reduce, not eliminate, major quantitative phagocyte and antibody deficits; absence of opportunistic infection makes profound T-cell failure less likely.
- Select pathway-level assessment with complement components and a validated functional assay, alongside evaluation for acquired complement consumption and the clinical infection pathway.
- Conclude that terminal complement deficiency is the leading mechanism requiring specialist confirmation, while immediate infection prevention and acute-care advice proceed through appropriate services.
- Verify by predicting the test pattern before results: impaired terminal pathway function with preserved upstream activation would fit; a global reduction should reopen consumption, liver production or sample issues.