The immune system is a complex set of cells and proteins that are intended to protect from infection and keep us healthy.
Natural barriers, such as the skin and mucus, are an effective first line of protection. But once germs get inside the body, white blood cells are the primary defense. Other proteins including antibodies and complement also play a role in host defense.
White blood cells are made in the bone marrow and travel through the bloodstream and lymph nodes. Problems with the quantity of white cells produced and how well they do their jobs make us more susceptible to certain germs, and can be the underlying cause of a primary immunodeficiency disease (PIDD).
There are several types of cells that defend and fight infections and disease.
There are two main types of lymphocytes: T-cells and B-cells.
T-cells are helpers, regulators and fighters. Some T-cells provide support to other cells to help them fight. Others alert cells when it is time to fight (and when to stop fighting). There are also T-cells that actually fight and kill infected cells in the body.
Diseases associated with T cells include:
• Severe Combined Immunodeficiency (SCID)
• DiGeorge Syndrome (DGS)
The primary role of B-lymphocytes (B-cells) is to make antibodies (also known as immunoglobulin or Ig). Antibodies recognize an organism as foreign and set off a complex chain of reactions involving other components of the immune system.
Classifications of types of immunoglobulins (antibodies) include:
• IgG - the main infection-fighting antibody type in our bloodstream
• IgA - helps to protect the gut and respiratory tract
• IgM - a preliminary response antibody, made at the early signs of infection
• IgE - fight against parasites, also responsible for triggering allergy symptoms
There are multiple types of primary immunodeficiency diseases caused by problems with B-cells and antibodies. Some of the most common are:
• Selective IgA Deficiency
• X-Linked Agammaglobulinemia
• Common Variable Immunodeficiency
Phagocytic cells have the ability to move from the bloodstream into tissues. Once at the site of infection, they ingest the invading microorganisms. Phagocytes are classified into neutrophils and monocytes.
Neutrophils or granulocytes are attracted to sites of inflammation, injury or infection. When they find these sites, they release chemicals such as hydrogen peroxide to kill germs or clean up wounds. These are the frontline fighters against infection, but they need to be continually renewed because they have a short life span in our blood stream.
Problems with neutrophils related to PIDD include:
• Chronic Granulomatous Disease (CGD)
Monocytes/macrophages circulate in the blood and become macrophages in the tissues. These cells are very important in alerting the immune system about an infection. Macrophages are scavengers whose job is to engulf or eat up infecting germs and even infected cells. Macrophages also help to overcome infection by secreting signals that help activate other cell types to fight against infections.
Problems with macrophages may involve disorders in the interleukin-12 (IL-12) or interferon-gamma pathways.
Learn more about PIDD.
This article has been reviewed by Thanai Pongdee, MD, FAAAAI