What is considered the most common primary immunodeficiency disorder?

The most common primary immunodeficiency disorder is selective IgA deficiency. This condition is characterized by an inability to produce adequate levels of immunoglobulin A (IgA), which is crucial for mucosal immunity. Individuals with selective IgA deficiency often experience increased susceptibility to infections, particularly respiratory and gastrointestinal infections, due to the lack of IgA in mucosal surfaces like the gut and respiratory tract.

Selective IgA deficiency is more prevalent than other primary immunodeficiencies and is frequently found when assessing individuals for immunological issues. It can sometimes go undiagnosed because many patients may not present with significant health problems and can maintain relatively normal health.

The other immunodeficiency disorders listed, while significant in their own right, tend to occur less frequently in the general population. For example, X-linked agammaglobulinemia primarily affects males and leads to a complete absence of immunoglobulins, while common variable immunodeficiency causes reduced immunoglobulin levels but is less common than selective IgA deficiency. Hyper-IgM syndrome is also a serious condition but is quite rare compared to selective IgA deficiency. Thus, selective IgA deficiency stands out as the most commonly encountered primary immunodeficiency.