10 warning signs for Children (up to age 18) :

1 - ≥ 4 new ear infections within 1 year

2 - ≥ 2 serious sinus infections within 1 year
3 - ≥ 2 pneumonias within 1 year

4 - ≥ 2 months on antibiotics with little effect

5 - Need for intravenous antibiotics to clear infections
6 - Failure of an infant to gain weight or grow normally

7 - Persistent thrush in mouth or fungal infection on skin

8 -Recurrent, deep skin or organ abscesses

9 - ≥ 2 deep-seated infections including septicemia

10 - A family history of Primary Immunodeficiency
* The warning signs for children are slightly different than those for adults 1. If someone you know is affected by two or more of these warning signs consult a physician about the possible underlying presence of a primary immunodeficiency.
Immunodeficiencies can have variable clinical symptoms. Many of these symptoms can suggest and underlying primary immunodeficiency, without it actually being the case. Other signs, like autoimmune diseases can also lead to a diagnosis.
1 Arkwright and Gennery, Ten warning signs of primary immunodeficiency: a new paradigm is needed for the 21st century, Ann. N.Y. Acad Sci. 1238 (2011) 714