Primary immunodeficiencies (PID) include >200 inherited disorders that affect the development and function of lymphocytes, neutrophils, eosinophils or monocytes. The first step in diagnosing a primary immunodeficiency is to suspect it. A good complete blood count (CBC) with differential count and peripheral smear is often the first investigation ordered when one suspects a PID. Trends in WBC counts and differentials are often more informative than a single abnormal value. Moreover, they should be always interpreted according to the age.
- Neutrophilia is observed in phagocytic disorders such as leukocyte adhesion defects and chronic granulomatous diseases. Neutrophilic leukocytosis that persists in between episodes of infections has been characteristically described in leukocyte adhesion deficiency.
- Neutropenia is a feature of several PIDs either as a result of decreased production of neutrophils or as a result of autoimmunity with increased destruction.
- Lymphopenia in a newborn or infant is perhaps the most important indicator for urgent referral to a higher centre. Often absolute lymphocyte count is ignored in a differential count as emphasis is mainly given to total WBC and neutrophil count. Most patients with severe combined immunodeficiency are lymphopenic at birth and they are uniformly fatal before 1 year of age unless definitive therapy is instituted.
- Lymphocytosis is usually a feature of viral infections and rarely points to PIDs.
- Persistent Eosinophilia is observed in primary immunodeficiencies including Hyper IgE syndromes.
- Monocytopenia also has been defined with primary immunodeficiencies with susceptibility to mycobacterial, fungal and viral infections.
- Platelet Counts – In PIDs, thrombocytopenia has been observed as a direct effect of the disease or as a feature of autoimmunity.
- Anemia is commonly observed in many immunodeficiencies as a result of repeated infections and malabsorption.