Immunological diseases occur when the immune system, which normally protects against infections, malfunctions. The most common types of immunological diseases are allergies, autoimmune diseases, and immune deficiencies.

An allergy is an abnormal reaction of the immune system to foreign substances, such as plant pollen, fungal spores, animal danders, medications, and foods. Rhus dermatitis is an allergy caused by contact with urushiol, an oil resin produced by poison ivy, poison oak, and poison sumac.

Autoimmune diseases develop when the immune system goes awry and attacks the body's own tissues. Autoimmune disorders includes lupus erythematosus, rheumatoid arthritis, juvenile-onset diabetes, and myasthenia gravis. The causes are unknown, although some scientists suspect the diseases may be triggered in some cases by a pathogen, such as a virus, or other environmental factor.

Immune deficiency diseases develop when the immune system becomes impaired, resulting in more common, frequent, or severe infections. The immune system may be damaged by a genetic abnormality or by illness, injury, the use of a strong drug such as those used in chemotherapy, or malnutrition.