Laboratory investigations - full blood count (FBC)

Role

In immunology/allergy practice, the FBC provides key clues via the differential white cell count, particularly eosinophils and lymphocytes, alongside general markers of chronic disease/marrow involvement
Eosinophilia and lymphocyte abnormalities are common incidental findings prompting immunology referral

Eosinophilia - allergic/atopic disease, drug reaction, parasitic infection, and (if marked/persistent) eosinophilic granulomatosis with polyangiitis (EGPA), hypereosinophilic syndrome, or haematological malignancy
Lymphopenia - can reflect immunodeficiency (primary or acquired, including HIV), corticosteroid effect, or active SLE (lymphopenia is one of the haematological criteria for SLE classification)
Lymphocytosis - viral infection (reactive) most commonly; persistent lymphocytosis in an older adult raises consideration of chronic lymphocytic leukaemia

The degree and persistence of an abnormality guides urgency - marked or persistent eosinophilia/lymphopenia beyond what a self-limited process would explain warrants further workup (lymphocyte subsets, immunoglobulins, specific serology) rather than repeat testing alone

Guides referral - marked eosinophilia with organ involvement (cardiac, pulmonary, cutaneous) needs urgent assessment for hypereosinophilic syndrome or EGPA rather than routine follow-up

Allergic disease, EGPA, hypereosinophilic syndrome, primary and acquired immunodeficiency, SLE

A single abnormal differential count is often transient/reactive - trend over serial testing, alongside the clinical picture, distinguishes a benign reactive change from one warranting further immunological workup

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