By Linda Harvan, FAUSA
Our immune system is a complex network of tissues, cells and organs working to prevent and limit illness and infections. The two main parts of the immune system work together – the innate immune system, with which you are born, and the adaptive immune system, which you develop.
The innate immune system is the body’s first line of defense. One part is physical barriers, such as skin, the clear layer over the cornea and specialized tissues lining the lungs, bladder and digestive system. Another part is chemical barriers, such as sweat, tears, gastric acid and mucous that can wash away germs but also have enzymes that kill bacteria. The innate system is general and the first to respond to a pathogen.
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Many types of white blood cells, or leukocytes, travel throughout the body in the circulatory system waiting to attack invaders. Phagocytes engulf and destroy bacteria and viruses. Macrophages can roam outside the circulatory system and release cytokines to signal an area with pathogens. Mast cells create an inflammatory response and release cytokines to recruit macrophages and neutrophils to the area of infection. Neutrophils contain granules that are very toxic to bacteria and fungi and cause them to stop multiplying or die on contact. A healthy adult makes approximately 100 billion new neutrophils per day, and because there are so many of them, they are usually the first to arrive at the site of an infection. Eosinophils release toxic proteins that kill bacteria and parasites, but also cause tissue damage so their use is regulated. Basophils release histamine and are a key player in allergic responses. Natural killer cells destroy infected host cells to prevent the spread of infection. Dendritic cells are found in tissues, which are common points of infection. They act as messengers between the innate and adaptive immune systems because they process and present antigen material.
While the innate immune system is general, fast-acting and present at birth, the adaptive immune system is specific, has a slower response and is developed over a person’s lifetime in response to exposure to foreign substances. The adaptive system is composed of B cells and T cells. The B lymphocytes are formed in the bone marrow and develop antibodies after exposure to a pathogen, which can take several days. After the first exposure, the immune system will recognize the same pathogen and defend against it in the future. Immunizations also train your immune system to make antibodies to specific diseases. T lymphocytes develop in the bone marrow but mature in the thymus, located in the upper chest behind the sternum. T cells help to destroy infected or cancerous cells.
When B cells encounter antigens, they divide into plasma cells and memory cells. The plasma cells produce antibodies, and the memory cells allow a rapid response when the same infection is encountered again. Antibodies are specialized, and each antibody fits only one antigen. Antibodies, or immunoglobins (Ig), are divided into five main classes: IgG, IgA, IgM, IgD, and IgE. IgG antibodies are the most common, last a few weeks, and are the only antibodies that cross the placenta and provide immunity from the mother to her newborn. IgA antibodies are found in secretions such as tears, saliva, mucous and bile and protect against infections of the respiratory tract and intestines. IgM antibodies are the first produced in response to an infection and provide early protection. Although the function of IgD is not completely understood, it is known that IgD signals B cells to be activated for immune defense. IgE antibodies produce an allergic response.
T cells also have different functions. “Killer” or cytotoxic T cells destroy infected cells. Killer T cells protect the body against certain bacteria and viruses, but also respond to foreign tissues such as transplanted organs. Helper T cells assist B cells to produce antibodies and assist killer T cells in their attack on foreign substances.1 Regulatory T cells act as a thermostat and suppress the immune response once an infection has been cured. They play a critical role in preventing autoimmunity.
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An individual with a healthy, functioning immune system has many weapons in the fight against bacteria, viruses, fungi and other pathogens. For individuals with Primary Immunodeficiency (PI), however, part of the immune system is missing or does not function properly. People with PI are more susceptible to infections for their entire lives. There are more than 400 primary immunodeficiencies, and each has different characteristics. People with Common Variable Immune Deficiency (CVID) have low levels of serum immunoglobins and antibodies. Some people have low levels of both IgG and IgA, others have low levels of all three of the major types (IgG, IgA, and IgM), and still others have defects of the T cells. My husband Tom has extremely low levels of IgG and makes no IgA. All other immunoglobin levels measured were normal.
In the next article, we will explore symptoms and warning signs of Primary Immunodeficiency, especially CVID.
Sources:
1 The Immune System and Primary Immunodeficiency | Immune Deficiency Foundation (primaryimmune.org)
Innate immunity (article) | Immune system | Khan Academy
Difference between Innate and Adaptive Immunity (microbiologyinfo.com)
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Immune System: Parts & Common Problems (clevelandclinic.org)
The Immune System | Johns Hopkins Medicine
Overview of the Immune System | NIH: National Institute of Allergy and Infectious Diseases
anti-Immunoglobulin D (IgD) secondary antibodies (antibodies-online.com)
Regulatory T cells (TREG) and their roles in immune system with respect to immunopathological disorders – PubMed (nih.gov)
The Immune System and Primary Immunodeficiency | Immune Deficiency Foundation (primaryimmune.org)
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