Immunodeficiency: warning signs and the infection pattern by defect !

Topic

When infections come too often, last too long or are caused by unusual germs, the immune system may be missing a piece.

Updated 2026-11-01

What it is

An immunodeficiency is a reduced or absent immune response. Primary (inborn) defects are rare and usually show up in childhood; secondary (acquired) ones from HIV, malnutrition, drugs or loss of the spleen are far more common. The kind of germ that keeps causing trouble points to the part of the immune system that fails, and that guides the first tests.

Key facts

Warning signs in children: several ear or sinus infections in one year, two or more pneumonias in a year, antibiotics for months with little effect, deep abscesses, persistent thrush, needing IV antibiotics, poor growth and a family history of immunodeficiency.
Warning signs in adults: repeated ear, sinus or lung infections, chronic diarrhea with weight loss, recurrent viral infections (herpes, warts), deep abscesses, persistent fungal infections and infection with atypical mycobacteria.
Two or more warning signs justify an evaluation by an immunologist (Jeffrey Modell Foundation criteria).
A severe infection from a germ that rarely harms healthy people (an opportunistic infection) is itself a clue.
Live vaccines (BCG, rotavirus, oral polio, MMR, varicella, yellow fever) can cause disease in severe T-cell defects; a BCG complication in an infant is a classic SCID clue.

The infection pattern by defect

Defect	Usual onset	Typical germs and clues	First tests
Antibody (B cell)	After about 6 months (maternal IgG fades); CVID in adults	Encapsulated bacteria (pneumococcus, Haemophilus), sinusitis, otitis, pneumonia, Giardia, enteroviruses	IgG, IgA, IgM; vaccine titers; CD19 B-cell count
T cell / combined	First weeks to months of life	Candida, Pneumocystis, CMV and other viruses, chronic diarrhea, failure to thrive, live-vaccine disease	Lymphocyte count; CD3, CD4, CD8 by flow cytometry; TREC newborn screen
Phagocyte	Infancy or childhood	Staphylococcus aureus, Aspergillus, Serratia, Nocardia; abscesses, poor pus, gingivitis	Neutrophil count; DHR oxidative burst test; CD18 by flow cytometry
Complement	Any age	Neisseria (terminal C5–C9); encapsulated bacteria (C3); lupus-like disease (C1q, C2, C4)	CH50 (classical), AH50 (alternative), C3, C4
Secondary (acquired)	Any age, often adults	Depends on cause: opportunistic infections in HIV, encapsulated bacteria after splenectomy	HIV test, glucose, albumin, drug history, smear for Howell-Jolly bodies

Clinical notes

Recurrent infections that all occur in one site (for example, one lung lobe) suggest an anatomic problem such as a foreign body or bronchiectasis, not an immunodeficiency.
In Colombia and Latin America, secondary causes (HIV, malnutrition, steroids, chemotherapy) are far more frequent than inborn errors, but inborn errors are underdiagnosed.
Infants with suspected severe T-cell deficiency should not receive live vaccines or non-irradiated blood products until the evaluation is complete.

Test yourself

Think of the answer, then tap the question to see it.

Why do children with X-linked agammaglobulinemia usually stay well for the first months of life?

Maternal IgG crosses the placenta and protects them until it fades, around 6 months of age; then recurrent bacterial infections begin.

A child has recurrent skin abscesses and a liver abscess due to Staphylococcus aureus. Which part of the immune system is suspected?

Phagocytes (neutrophils), for example chronic granulomatous disease; an oxidative burst test (DHR) is the screening test.

Recurrent meningococcal infections point to a deficiency of which complement components?

The terminal components C5 to C9 (the membrane attack complex), screened with the CH50.

What does an infant with chronic diarrhea, thrush, failure to thrive and Pneumocystis pneumonia suggest?

A severe T-cell or combined immunodeficiency such as SCID.

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Microorganisms

← All of immunology

Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.

Sources: 10 Warning Signs of PI (Jeffrey Modell Foundation, 2026); Immunodeficiency diseases (MedlinePlus, U.S. National Library of Medicine, 2026).

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