Immunodeficiency disorder is a condition where the defense mechanisms of the body against different microbial infections are weakened. This deficiency makes the body vulnerable to bacterial and viral attacks. Deficiencies of defense mechanism may involve specific immune functions like humoral and cell mediated immunity or non specific such as complement and phagocytosis. Immune disorders are classified into primary and secondary.
1. Primary deficiencies are developed from an inability in the development of immune mechanism by birth.
2. Secondary immune deficiencies are acquired as a consequence of certain diseases, drugs or other process which affects the normal functioning of the mature immune system.  
Primary Immunodeficiency
The established types of Primary immune deficiencies are listed below.
Humoral immunodeficiencies
1) X-linked agammaglobulinemia (XLA); The first humoral immunodeficiency disorder, recognized by Brutan (1952).
2) Transient hypogammaglobulinemia of infancy: This is due to an abnormal delay in the initiation of Ig G in some infants.
3) Common variable immunodeficiency: characterized by recurrent pyogenic infections and increased incidence of autoimmune disease.
4) Selective immunoglobulin deficiencies: Selective deficiency of one or more immunoglobulin classes results in this deficiency. IgA deficiency is more relevant.
IgM deficiency leads to septicemia and deficiency of IgG subclasses have been observed with chronic progressive bronchiectasis (enlargement of the airways of the lungs).
5) Immunodeficiencies with hyper- IgM: Elevated IgM level and low IgA and IgG levels is observed.
6) Trans-cobalamine II deficiency: Deficiency of Vitamin B12 is seen which results in depleted plasma cells, diminished immunoglobulin levels and impaired phagocytosis.
I. Cellular Immunodeficiency
1) DiGeorge’s syndrome or Thymic hypoplasia: It is a developmental defect involving pharyngeal pouches. This results in aplasia, or hypoplasia of the thymus and parathyroid glands.
Cell mediated immunity is primarily involved.
2) Chronic mucocutaneous candidiasis: In this condition the cell mediated immunity of the body to Candida albicans becomes weak and the patients develop chronic candidiasis of the mucosa, skin and nails.
3) Purine nucleoside phosphorylase (PNP) deficiency: This enzyme deficiency degrades the purine to hypoxanthine and finally to uric acid.  
II. Combined immunodeficiencies
1) Ataxia telangiectasia: It is a hereditary condition, where combined immunodeficiency is linked with lack of coordination of cerebellar muscle, and other chromosomal abnormalities.
2) Wiskott-Aldrich syndrome: Cell mediated immunity undergoes progressive worsening associated with cellular depletion of thymus and lymph nodes.
3) Immunodeficiency with thymoma: occurs in adults with a benign thymic tumor and impaired immunity.
4) Immunodeficiency with short-limbed dwarfism: This defect is apparently inherent in nature.
5) Episodic lumphopenia with lymphocytotoxin: In this condition the patient lack immunological memory and so the secondary antibody response is abolished.
III. Disorders of Complement: This includes complement component deficiencies and complement inhibitor deficiencies.
IV. Disorder of phagocytosis: The mechanism of phagocytosis may be impaired causing Chronic granulomatous disease, Chediak-Higashi syndrome (genetic disorder), Job’s syndrome, Shwachman’s disease (decreased neutrophil mobility) etc.
Secondary immunodeficiency
These are more common than primary deficiencies. Different factors such as malnutrition, malignancy, infections, metabolic disorders and cytotoxic drugs are involved. AIDS is an example.
Treatment
Antibiotics and replacement of antibody are followed. For viral disorders, interferons are used.
Prevention
Primary diseases can only be controlled and treated. Secondary disorder can be prevented by avoiding unprotected sex with  the virus carriers in the case of AIDS.