When a pattern of frequent infections suggests an immunodeficiency, the doctor begins by exploring the patient and family history, and then conducts a physical examination.

• The patient’s history. What infections has the patient had in the past, or has now? Have they been unusually frequent, or severe, or long-lasting? Have they failed to respond to standard treatments? When a child who is immunologically normal develops a string of infections, they are usually mild and short-lived, and between infections the child recovers completely.

What, besides a PI, might explain the high rate of infections? Normal immune responses can be suppressed by many factors, including malnutrition, injuries such as burns, and certain types of drugs (corticosteroids, for instance). Immune responses can also be muted by some diseases, such as leukemia, and some infections, including infectious mononucleosis (mono), measles, chicken pox, and AIDS. In fact, almost every serious illness impairs the immune responses.

• Physical examination: Is the child well-nourished and growing well? A severely immunodeficient child is likely to look sickly and pale. Very often the child is underweight and lags behind in growth and development.

The child may be shy or quiet. An active, robust, healthy-looking child is less likely to have a serious immune deficiency. The doctor will listen for changes in the lungs and look for rashes, sores, thrush in the mouth, an enlarged spleen or liver, and swollen joints. Some immunodeficient children may lack palpable tonsils or lymph nodes in the neck.

• Family history. Have any family members or relatives ever been diagnosed with PI or shown an unusual susceptibility to infections? Have there been any infant deaths from infections? Were only boys affected?

To find out if illness can be traced to an immunodeficiency, laboratory tests are necessary. These tests, most of which can be performed on a sample of blood, probe the soundness of the various parts of the immune system. Are all the right immune cells present, in adequate numbers, and are they working properly? Are there normal amounts and types of antibodies?

Screening starts out with a few relatively simple and inexpensive routine tests. In fact, just two routine tests—complete blood count and quantitative immunoglobulins—will detect most, but not all, immunodeficiencies.

If antibodies are normal, or if the patient’s infections seem to be caused by viruses or fungi, the T cells should be checked.If the T cells are present in normal numbers and function normally, phagocyte function should be evaluated.

When screening tests indicate an immunodeficiency, or when they fail to explain a stubborn infection, additional tests will likely be needed. There are dozens of sophisticated tests that allow doctors to identify and count subsets of B cells and T cells, and to assess subtle abnormalities in antibodies, immune cells, and immune tissues. Tests can also probe the characteristics of infectious germs.