Hypogammaglobulinemia can affect children and adults. There are two main types:

Primary (congenital) hypogammaglobulinemia: Several rare genetic disorders called primary immunodeficiency disease (PIDD) cause low immunoglobulin. With PIDD, you’re born with a mutation, or error, in genetic material that tells your immune system how to work. The mutations often run in biological families.

Secondary (acquired) hypogammaglobulinemia: Medical conditions and medications can cause low immunoglobulin levels. Most people with hypogammaglobulinemia get it because of a condition or medication side effect.

Hypogammaglobulinemia is having low levels of antibodies called immunoglobulins in your immune system. Your immune system protects you from germs like viruses and bacteria that make you sick. It includes immune cells, like B-cells, that defend you from germs and fight disease. When a germ enters your body, B-cells make immunoglobulins to destroy it.

You may learn you have hypogammaglobulinemia when you get blood test results.

The word provides clues about what’s involved:

Hypo: Below normal.

Gamma globulins: Most common type of immunoglobulin.

Emia: In your blood.

Low levels of immunoglobulins weakens your body’s defenses against invaders. It increases risk of infections and other illnesses.

Myeloma as well as it's treatments can cause low gamma globulin.

Other PIDDs that cause primary hypogammaglobulinemia include:
Ataxia-telangiectasia.
Autosomal recessive agammaglobulinemia (ARA).
Good syndrome (GS).
Isolated non-IgG immunoglobulin deficiencies.
Severe combined immunodeficiency (SCID).
Specific antibody deficiency (SAD).
Wiskott-Aldrich syndrome (WAS).
Secondary hypogammaglobulinemia
Acquired causes of hypogammaglobulinemia include:

Medications: Various drug types can cause low immunoglobulin levels; immunosuppressants, anti-seizure medication, corticosteroids and chemotherapy drugs.

Nephrotic syndrome: This condition causes your kidneys to release too much protein in your urine.

Protein-losing enteropathy: This condition causes you to lose too much protein through your GI tract.

Cancer: Cancers that affect your B-cells can cause low immunoglobulin levels. Types include chronic lymphocytic leukemia, lymphoma and multiple myeloma.

What causes hypogammaglobulinemia?
Your immune system may not make enough immunoglobulins, or something destroys them. Causes differ depending on whether you have primary or secondary hypogammaglobulinemia.

Primary hypogammaglobulinemia, Some of the PIDDs most commonly associated with hypogammaglobulinemia include:

Common variable immunodeficiency (CVID): Various gene mutations prevent your body from making enough immunoglobulin. CVID is the most common cause of primary hypogammaglobulinemia in adults.
X-linked agammaglobulinemia (XLA): A mutation on X chromosome prevents your body from making enough B-cells or immunoglobulins. XLA primarily affects babies assigned male at birth (as they only have one X chromosome). XLA is a common cause of hypogammaglobulinemia in children.
Transient hypogammaglobulinemia of infancy (THI): Immunoglobulin levels start falling at around 3 months old. Infants with THI may develop infections, but often they don’t show symptoms. THI usually resolves on its own before age 3.
Selective IgA deficiency: Your body doesn’t produce enough of a specific type of immunoglobulin, immunoglobulin A.
Hyper-IgM syndromes: Your body produces normal or increased levels of some types of immunoglobulin but not enough of others.