This document provides an overview of immunodeficiency diseases. It describes how immunodeficiencies can be primary, due to abnormalities in immune system development, or secondary, resulting from other diseases or conditions. The major classifications of primary immunodeficiencies are then outlined, including humoral deficiencies affecting B cells, cellular deficiencies affecting T cells, combined deficiencies, and disorders of complement and phagocytosis. Several specific primary immunodeficiency diseases are then described in more detail. Secondary immunodeficiencies resulting from external factors like malnutrition, infection, or drugs are also briefly discussed.