- Immunodeficiency Diseases are those in which certain functionalities of the immune response are either reduced or absent. These diseases usually manifest as increased susceptibilities to certain microbial infections. Immunodeficiency diseases which are inherited or appear sporadically are termed "Primary" while those which are due to acquired damage to the immune system are termed "Secondary".
- The specific clinical consequences of any immunodeficiency disease depend on the particular aspects of immunity which are defective. However, several general patterns can be gleaned depending on whether the defect primarily affects the Humoral Immunity, Phagocytes, or Cell-mediated Immunity.
- Humoral Immunity Defects
- Defects of Humoral Immunity result in an inability to generate antigen-specific antibody to microbes. This manifests as an increased susceptibility to certain pyogenic bacteria, especially Haemophilus influenzae, Streptococcus pneumoniae, and Staphylococcus aureus. Although these patients can generally clear viruses, due to an intact Cell-mediated immune response, they often cannot establish long-term immunity due to an inability to generate neutralizing antibodies to the virus. Consequently, such patients will display repeated infections to viruses.
- Phagocyte Defects
- Under this category we include defects in phagocyte function which are critical aspects of the Innate Immune Response. Because phagocytosis is one of the primary mechanisms by which bacteria are cleared, the primary manifestation of phagocytic defects is usually a susceptibility to pyogenic bacteria.
- Cell-mediated Immunity Defects
- Defects of Cell-mediated Immunity severely limit the host response to viruses and fungi. Consequently, patients tend to display repeated and severe infections with these classes of microbes. Additionally, because Cell-mediated immunity is critical for regulation of humoral immunity, such patients sometimes display some defects in antibody responses as well.
- Primary Immunodeficiencies are rare and the vast majority are due to inherited genetic lesions in proteins critical to the development or proper functioning of immune cells. These immunodeficiencies often manifest early in life, during infancy or childhood.
- Defects of Phagocytosis
- Defects of Lymphocytes
- Other Defects
- A wide variety of acquired pathogenic processes can lead to impairment of immunity and are thus considered Secondary Immunodeficiency Diseases. One of the most common causes of secondary immunodeficiency is naturally iatrogenic therapy with immunosuppressive drugs. In this section we specifically refer the reader only to the Acquired Immunodeficiency Syndrome page.