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What is Immunodeficiency and explain in detail?
Immunodeficiency, also known as immunocompromisation, is a state in which the immune systems ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that affect the patients immune system. Examples of these extrinsic factors include HIV infection and environmental factors, such as nutrition.
Immunocompromisation may also be due to genetic diseases/flaws such as SCID.
In clinical settings, immunosuppression by some drugs, such as steroids, can either be an adverse effect or the intended purpose of the treatment. Examples of such use is in organ transplant surgery as an anti-rejection measure and in patients with an overactive immune system, as in autoimmune diseases. Some people are born with intrinsic defects in their immune system, or primary immunodeficiency.A person who has an immunodeficiency of any kind is said to be immunocompromised. An immunocompromised individual may particularly be vulnerable to opportunistic infections, in addition to normal infections that could affect anyone. It also decreases cancer immunosurveillance, in which the immune system scans the bodys cells and kills neoplastic ones. They are also more susceptible to infectious diseases owing to reduced protection afforded by vaccines.

Types
By affected component
Humoral immune deficiency (including B cell deficiency or dysfunction), with signs or symptoms depending on the cause, but generally include signs of hypogammaglobulinemia (decrease of one or more types of antibodies) with presentations including repeated mild respiratory infections, and/or agammaglobulinemia (lack of all or most antibody production) which results in frequent severe infections and is often fatal.
T cell deficiency, often causes secondary disorders such as acquired immune deficiency syndrome (AIDS).
Granulocyte deficiency, including decreased numbers of granulocytes (called as granulocytopenia or, if absent, agranulocytosis) such as of neutrophil granulocytes (termed neutropenia). Granulocyte deficiencies also include decreased function of individual granulocytes, such as in chronic granulomatous disease.
Asplenia, where there is no function of the spleen
Complement deficiency is where the function of the complement system is deficientIn reality, immunodeficiency often affects multiple components, with notable examples including severe combined immunodeficiency (which is primary) and acquired immune deficiency syndrome (which is secondary).

Primary or secondary
The distinction between primary versus secondary immunodeficiencies is based on, respectively, whether the cause originates in the immune system itself or is, in turn, due to insufficiency of a supporting component of it or an external decreasing factor of it.

Primary immunodeficiency
A number of rare diseases feature a heightened susceptibility to infections from childhood onward.  Primary Immunodeficiency is also known as congenital immunodeficiencies.  Many of these disorders are hereditary and are autosomal recessive or X-linked. There are over 95 recognised primary immunodeficiency syndromes; they are generally grouped by the part of the immune system that is malfunctioning, such as lymphocytes or granulocytes.The treatment of primary immunodeficiencies depends on the nature of the defect, and may involve antibody infusions, long-term antibiotics and (in some cases) stem cell transplantation. The characteristics of lacking and/or impaired antibody functions can be related to illnesses such as X-Linked Agammaglobulinemia and Common Variable Immune Deficiency

Secondary immunodeficiencies
Secondary immunodeficiencies, also known as acquired immunodeficiencies, can result from various immunosuppressive agents, for example, malnutrition, aging, particular medications (e.g., chemotherapy, disease-modifying antirheumatic drugs, immunosuppressive drugs after organ transplants, glucocorticoids) and environmental toxins like mercury and other heavy metals, pesticides and petrochemicals like styrene, dichlorobenzene, xylene, and ethylphenol. For medications, the term immunosuppression generally refers to both beneficial and potential adverse effects of decreasing the function of the immune system, while the term immunodeficiency generally refers solely to the adverse effect of increased risk for infection.
Many specific diseases directly or indirectly cause immunosuppression. This includes many types of cancer, particularly those of the bone marrow and blood cells (leukemia, lymphoma, multiple myeloma), and certain chronic infections. Immunodeficiency is also the hallmark of acquired immunodeficiency syndrome (AIDS), caused by the human immunodeficiency virus (HIV). HIV directly infects a small number of T helper cells, and also impairs other immune system responses indirectly.
Various hormonal and metabolic disorders can also result in immune deficiency including anemia, hypothyroidism and hyperglycemia.
Smoking, alcoholism and drug abuse also depress immune response.
Heavy schedules of training and competition in athletes increases their risk of immune deficiencies.

Diagnosis
Patients with immune deficiencies can present with variable clinical phenotypes. This often translates into a significant delay in their diagnosis, and resultant patient morbidity. A structured approach on when to suspect an immunodeficiency and the initial investigations pathway is given in the publication by Grammatikos et al.

Immunodeficiency and autoimmunity
There are a large number of immunodeficiency syndromes that present clinical and laboratory characteristics of autoimmunity. The decreased ability of the immune system to clear infections in these patients may be responsible for causing autoimmunity through perpetual immune system activation.
One example is common variable immunodeficiency (CVID) where multiple autoimmune diseases are seen, e.g., inflammatory bowel disease, autoimmune thrombocytopenia, and autoimmune thyroid disease.
Familial hemophagocytic lymphohistiocytosis, an autosomal recessive primary immunodeficiency, is another example