Human immune system consists of spleen, tonsils, bone marrow and lymph nodes; these organs are responsible for combating invading microorganisms (bacteria, viruses and parasites) as well as any harmful cellular mutations such as cancerous transformations.

Immunodeficiency disorders are medical conditions which prevent the body from fighting infection and diseases; it's of two types:

• Primary: it's also called congenital immunodeficiency disorders, as it develops since birth due to genetic and chromosomal causes.
• Secondary: it's also called acquired immunodeficiency disorders, as it develops later in life due to exposure to toxic substances or infection.

Immunodeficiency disorders develop due to the following factors:

1. Positive family history (especially in cases of primary type).
2. Decreased functional capacity of one of the constituents of the immune system, as in cases of spleen or tonsils removal or bone marrow depression.
3. Aging; as we get older, the immune system weakens.
4. Low protein intake in diet.
5. Lack of sleep.
6. Immunosuppressive drugs especially chemotherapy.