01Core principlesThe concepts and mechanisms needed to understand the subject.
The immune system must detect danger while limiting injury to self. Epithelia, mucus, antimicrobial molecules and resident cells form anatomical and chemical barriers. Pattern-recognition receptors detect conserved microbial structures and damage signals, activating phagocytosis, cytokines and local inflammation. Neutrophils provide rapid killing, macrophages ingest material and coordinate repair, dendritic cells connect tissue sensing to adaptive priming, and natural killer cells respond to stressed cells with altered inhibitory and activating signals.
Adaptive immunity adds enormous receptor diversity and memory. Each lymphocyte clone expresses a receptor with particular specificity generated during development. Antigen and appropriate co-stimulation select clones to proliferate and differentiate. CD4 T cells organise immune programmes through cytokines and cell contact; CD8 T cells can kill infected or abnormal cells presenting peptide on MHC I. B cells can become plasma cells that secrete immunoglobulin. Class switching changes antibody effector function without changing antigen specificity; affinity maturation selects improved binding within germinal centres.
Complement is a regulated proteolytic network. Classical activation can begin with antibody-containing complexes, lectin activation recognises carbohydrate patterns, and the alternative pathway amplifies activation on poorly protected surfaces. All form C3 convertases. C3b promotes opsonisation and contributes to downstream convertase formation; small fragments recruit and activate inflammatory cells; terminal components can form a membrane pore. Host regulators restrain amplification, so deficiency or dysregulation can produce infection susceptibility or inflammatory injury depending on the component.
Immune disease follows mechanism. Immunodeficiency may impair barrier, phagocyte, complement, antibody or cellular function, producing characteristic but overlapping infection patterns; acquired causes are common. Autoimmunity reflects failed tolerance plus tissue-specific expression and effector damage. Allergy may arise from IgE-mediated mast-cell activation, but not every adverse reaction is allergic. Laboratory tests sample parts of a dynamic network: timing, treatment, infection and pre-test probability influence results. Mechanistic reasoning prevents one abnormal antibody titre from replacing the patient's phenotype.
Key points
- 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.
- Complement converges on C3 activation, producing opsonisation, inflammatory signalling and, for susceptible targets, membrane attack complex formation.
- Central and peripheral tolerance reduce self-reactivity; breakdown can involve genetic susceptibility, tissue damage, altered regulation and environmental triggers.
- Hypersensitivity mechanisms overlap clinically: immediate IgE-driven, antibody-mediated, immune-complex and delayed T-cell responses describe mechanisms rather than isolated diseases.
- Interpret antibodies, complement and cell counts in clinical and temporal context; a positive immune marker is not automatically pathogenic or diagnostic.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Skin or mucosal disruption bypasses the first defence and changes organism access without necessarily indicating a primary immune-cell defect.
Deep bacterial or fungal infection, poor pus formation or impaired wound response can suggest quantitative or functional phagocyte problems, but acquired neutropenia is common.
Recurrent encapsulated bacterial respiratory infection and weak vaccine responses can indicate humoral dysfunction, while total immunoglobulin alone may not show functional specificity.
Persistent viral, fungal or opportunistic infection and failure to thrive can reflect cellular immune dysfunction, requiring urgent age- and context-specific assessment.
Recurrent invasive Neisseria infection suggests terminal pathway vulnerability, while early classical component problems can associate with immune-complex autoimmunity.
Autoantibodies gain meaning when phenotype, titre or specificity and tissue injury align; low-level positivity can occur without autoimmune disease.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Full blood count with differential - Why
- Quantify major circulating leukocyte populations and identify cytopenia or eosinophilia.
- Interpretation and limitations
- Use age-specific ranges and trends; normal counts do not prove normal cell function, and infection or medicines can cause secondary changes.
- 02
Quantitative immunoglobulins - Why
- Measure major immunoglobulin classes when humoral deficiency or paraprotein disease is suspected.
- Interpretation and limitations
- Interpret with age, protein loss and treatment; normal total concentrations do not establish adequate antigen-specific responses.
- 03
Antigen-specific antibody response - Why
- Assess functional humoral response to defined previous vaccination or exposure through specialist protocols.
- Interpretation and limitations
- Timing, prior doses and assay thresholds matter; do not improvise challenge schedules or infer broad competence from one antigen.
- 04
Lymphocyte subset enumeration - Why
- Quantify major T-cell, B-cell and natural-killer-cell populations in suspected cellular immune disorders.
- Interpretation and limitations
- Counts require age- and context-specific ranges and do not fully measure function; acute illness and treatment can transiently alter subsets.
- 05
Complement components and functional pathway assay - Why
- Assess consumption or deficiency in complement-mediated disease patterns.
- Interpretation and limitations
- Low components may reflect activation, reduced production or loss; functional patterns and repeat well-state samples help localise a persistent defect.
- 06
Targeted autoantibody testing - Why
- Support a phenotype-defined autoimmune hypothesis and refine disease associations.
- Interpretation and limitations
- Choose tests sequentially from pre-test probability; titre, pattern and specificity matter, and positivity alone does not prove tissue injury.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked exampleLocalise an immune-pathway defectInputs: a young adult has two episodes of invasive meningococcal disease, normal neutrophil count, normal quantitative immunoglobulins and no history of opportunistic infection.+
- 1Map 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.
- 2Use existing normal data to reduce, not eliminate, major quantitative phagocyte and antibody deficits; absence of opportunistic infection makes profound T-cell failure less likely.
- 3Select pathway-level assessment with complement components and a validated functional assay, alongside evaluation for acquired complement consumption and the clinical infection pathway.
- 4Conclude that terminal complement deficiency is the leading mechanism requiring specialist confirmation, while immediate infection prevention and acute-care advice proceed through appropriate services.
- 5Verify 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.
02Autoantibody reasoningInterpret a positive ANA in contextA low-titre ANA is reported after testing for nonspecific fatigue without organ features.+
- 1Reconstruct the pre-test hypothesis and check whether the phenotype contains objective features of a systemic autoimmune disease.
- 2Confirm assay, titre and pattern rather than treating the word positive as uniform evidence.
- 3Avoid indiscriminate antibody panels; perform targeted clinical review and tests only if a defined phenotype emerges.
- 4Explain that low-level autoantibodies can occur without disease and set follow-up by symptoms rather than serial testing alone.
03Immune mechanismTrace an immediate allergic responseSymptoms begin within minutes of exposure and involve urticaria, wheeze and hypotension.+
- 1Recognise a time-critical systemic reaction and prioritise emergency management through the current anaphylaxis pathway.
- 2Link sensitisation to allergen-specific IgE bound to mast cells, with re-exposure cross-linking receptors and releasing mediators.
- 3Separate acute clinical diagnosis from later specialist investigation; a delayed or negative marker does not replace the observed syndrome.
- 4Document exposure and timing precisely and arrange prevention planning after recovery.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Trend immune results against infection, treatment, protein loss and clinical state rather than labelling a transient abnormality as a fixed deficiency.
- Record organism, site, severity and microbiological confirmation for recurrent infections; frequency without phenotype is a weak immune history.
- After vaccination or replacement therapy, assess the specific clinical or functional outcome at the interval defined by the specialist protocol.
- Reassess an autoimmune hypothesis when organ pattern changes, while avoiding repetitive broad antibody screens for unchanged nonspecific symptoms.
- For suspected inherited immune disease, coordinate family history, molecular findings and functional phenotype because none is sufficient alone.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Innate and adaptive systems cooperate
Antibody recruits complement and phagocytes, while dendritic-cell innate sensing primes T cells; the two-system division is useful but not a biological wall.
Class switching preserves specificity
A B-cell clone can move from IgM to IgG, IgA or IgE effector functions while retaining recognition of the same antigenic target.
Consumption differs from deficiency
Low complement during active immune-complex disease may reflect use, whereas a reproducible pathway defect in a well state suggests inherited or persistent deficiency.
Memory is selective
Prior response accelerates recognition of the same antigen, but immune escape, waning and impaired hosts mean memory is neither universal nor absolute.
Immune markers are conditional evidence
An antibody result becomes clinically persuasive through specificity, titre, phenotype and timing; population screening can generate more incidental positives.
07Common pitfallsFrequent interpretation and management errors.
- 01
Describing innate immunity as wholly nonspecific when its receptors recognise defined conserved molecular patterns.
- 02
Equating a normal leukocyte count with normal phagocyte, lymphocyte or antibody function.
- 03
Treating all hypersensitivity as IgE-mediated or using the four-type scheme as a substitute for the actual clinical mechanism.
- 04
Interpreting low complement as inherited deficiency without considering activation, loss, synthesis and sample handling.
- 05
Ordering broad autoantibody panels in a low-probability setting and then allowing incidental positivity to create a diagnosis.
- 06
Using one immune abnormality to explain every infection without checking organism, site, exposure and acquired causes.