Diseases and Disorders of the Immune System

Immunodeficiencies

Immunodeficiency is a state where the immune system’s ability to fight infectious disease is impaired or absent.

Learning Objectives

Describe immunodeficiencies of the immune system

Key Takeaways

Key Points

  • A person with an immunodeficiency of any kind is said to be immunocompromised.
  • Primary immunodeficiencies are also known as congenital immunodeficiencies as these disorders are inherited.
  • Secondary immunodeficiencies are acquired from various environmental factors such as malnutrition, immunosuppressive drugs, and aging.

Key Terms

  • primary immunodeficiency: Any of the many genetic disorders due to an aspect of the body’s immune system being either absent or failing to function normally.

Immunodeficiency (or immune deficiency) is a state in which the immune system’s ability to fight infectious disease is compromised or absent entirely. Immunodeficiency may also decrease cancer immunosurveillance. Most cases of immunodeficiency are acquired (secondary) but some people are born with a defective immune system, or primary immunodeficiency. As an anti-rejection protocol, transplant patients take medications to suppress their immune system, as do some patients suffering from an over-active immune system. A person with an immunodeficiency of any kind is said to be immunocompromised. An immunocompromised person may be particularly susceptible to opportunistic infections, as well as normal infections affecting the general population.

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Genetic Immodeficiencies: Chart showing locations of malfunction leading to common errors of lymphocyte development.

Types of Immunodeficiencies

Humoral immune deficiency (with signs or symptoms depending on the cause) generally includes 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) resulting in frequent severe infections and is often fatal.

T-cell deficiency is often caused secondary disorders such as acquired immune deficiency syndrome.

Granulocyte deficiency refers to decreased numbers of granulocytes (called 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 is a granulocyte deficiency in which the spleen does not function.

Complement deficiency is where the function of the complement system is deficient.

Primary vs. Secondary

Immunodeficiency often affects multiple components, with notable examples including severe combined immunodeficiency (which is primary) and acquired immune deficiency syndrome (which is secondary). Distinctions between primary versus secondary immunodeficiencies are based, respectively, on whether the cause originates within the immune system itself or, in turn, is due to insufficiency of a supporting component of it, or an external decreasing factor of it.

A number of rare diseases feature a heightened susceptibility to infections from childhood onward. Primary Immunodeficiency is also known as congenital immunodeficiency. Many of these disorders are hereditary and are autosomal recessive or X-linked. There are over 80 recognized primary immunodeficiency syndromes—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.

Secondary immunodeficiencies, also known as acquired immunodeficiencies, can result from various immunosuppressive agents, for example, malnutrition, aging, and particular medications (e.g., chemotherapy, disease-modifying antirheumatic drugs, immunosuppressive drugs after organ transplants, and glucocorticoids). For medications, the term immunosuppression generally refers to both beneficial and potential adverse effects of decreasing the function of the immune system. 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 (those with leukemia, lymphoma, multiple myeloma, etc.), 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.

Autoimmune Diseases

Autoimmune diseases are an inappropriate immune response against tissues in the body.

Learning Objectives

Describe autoimmune diseases

Key Takeaways

Key Points

  • Autoimmune diseases arise from an immune system that does not function properly.
  • The common cause hypothesis of autoimmune diseases describes how autoimmune diseases are triggered by genes located in the same regions on the chromosome.
  • Despite genetic factors being involved in the onset of autoimmune disease, much is not know about the other causes of autoimmunity.

Key Terms

  • autoimmune diseases: Arise from an inappropriate immune response of the body against substances and tissues normally present in the body.
  • common cause hypothesis: Predicts that common disease-causing alleles, or variants, will be found in all human populations which manifest a given disease.

Autoimmune diseases arise from an inappropriate immune response of the body against substances and tissues normally present in the body. In other words, the immune system mistakes some part of the body as a pathogen and attacks its own cells. Autoimmune diseases are commonly considered complex immune disorders. While many autoimmune diseases are rare, collectively these diseases afflict millions of patients. Approximately 5–8% of the U.S. population suffers from this group of chronic, debilitating diseases. Despite their clinical diversity, they have one similarity, namely the dysfunction of the immune system. It is suspected that genetic defects play a role in the etiology of these diseases.

Causes and Treatments

Modern high throughput technologies, like mRNA microarrays, have enabled researchers to investigate diseases at a genome-wide level. In contrast to classical inherited genetic diseases, like sickle cell anemia, autoimmune diseases are not caused by the defect of a single gene, but by the dysfunction of the complex interaction of a group of genes. Although no autoimmune disease has been completely analyzed, there has been tremendous success in recent years in identifying major players in the development of autoimmune diseases. There are over 50 publications that list gene variants that are associated with a certain autoimmune disease. Interestingly, a lot of these genes are located in the same regions on the chromosomes, the so called susceptibility regions. This has led to a “common cause hypothesis” of autoimmune disorders. Several organizations and institutes have established programs to investigate this common cause hypothesis. However, defects of one or more of these genes do not cause an autoimmune disease, but only predispose a person for an autoimmune disease.

The factors that trigger an autoimmune disease are still unknown. Studies with monogenetic twins have revealed that genetic influences only account for 25–40% of the disease risk making gene-environment interactions or environmental influences the predominant factors. The environmental influences are very diverse, rendering research in this area extremely difficult. These influences may be toxic substances like mercury in one case and ultraviolet light or even certain nutrients in another. Moreover, several bacteria, viruses, or hormones are among the suspected triggers of autoimmune disorders. The treatment of autoimmune diseases is typically with immunosuppression: medication which decreases the immune response.

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Autoimmune Response: Erythema multiform is a rash that appears as an auto-immune response to a predisposing agent: a bacteria, an immunization, an antibiotic (in this case, amoxicillin).

Infectious Mononucleosis

Mononucleosis is an infectious disease caused by the Epstein-Barr virus (EBV) and results in flu-like symptoms.

Learning Objectives

Describe infectious mononucleosis

Key Takeaways

Key Points

  • Infectious mononucleosis is an infectious disease caused by the Epstein-Barr virus (EBV).
  • Infectious mononucleosis (also called mono or kissing disease) is spread orally and is characterized by symptoms such as fever, sore throat, and fatigue.
  • Once infected, the virus lives dormantly in B lymphocytes in a person.

Key Terms

  • Epstein-Barr virus (EBV): Virus responsible for causing infectious mononucelosis.
  • infectious mononucleosis: An infectious, widespread viral disease caused by the Epstein–Barr virus (EBV), one type of herpes virus. Most people are exposed to the virus as children, when the disease produces no noticeable or only flu-like symptoms.

Infectious mononucleosis is an infectious, widespread viral disease caused by the Epstein–Barr virus (EBV), one type of herpes virus, to which more than 90% of adults have been exposed. Occasionally, the symptoms can recur at a later period. It is sometimes colloquially known as the “kissing disease” from its oral transmission. Most people are exposed to the virus as children, when the disease produces no noticeable or only flu-like symptoms. In developing countries, people are exposed to the virus in early childhood more often than in developed countries. As a result, the disease in its observable form is more common in developed countries. It is most common among adolescents and young adults.

Symptoms

The disease is characterized by fever, sore throat, and fatigue, along with several other possible signs and symptoms, especially in adolescents and young adults. The infection is spread via saliva and has an incubation period of four to seven weeks. Symptoms usually persist for two to three weeks, but fatigue is often more prolonged. Infectious mononucleosis is primarily diagnosed by observation of symptoms, but suspicion can be confirmed by several diagnostic tests. The most commonly used diagnostic criterion is the presence of 50% lymphocytes with at least 10% atypical lymphocytes (large, irregular nuclei), while the person also has fever, pharyngitis, and adenopathy. Infectious mononucleosis is generally self-limiting, so only symptomatic and/or supportive treatments are used. Rest is recommended during the acute phase of the infection.

Once the acute symptoms of an initial infection disappear, they often do not return. But once infected, the patient carries the virus for the rest of his or her life. The virus typically lives dormantly in B lymphocytes. Independent infections of mononucleosis may be contracted multiple times, regardless of whether the patient is already carrying the virus dormantly.

This image contains the main symptoms of mononucleosis according to parts of the body. For example, soreness and redness are the main throat symptoms.

Main Symptoms of Infectious Mononucleosis: This graph illustrates the location of the symptoms.

Psoriasis

Psoriasis is an autoimmune disease that affects the skin, caused by faulty immune signals that speed up the growth cycle of skin cells.

Learning Objectives

Describe psoriasis

Key Takeaways

Key Points

  • The plaque form is most common form of psoriasis and usually presents as red and white hues of scaly patches on the epidermis.
  • Psoriasis is a chronic recurring condition that varies in severity from minor, localized patches to complete body coverage.
  • The cause of psoriasis is not fully understood, but it is believed to have a genetic component.
  • There is currently no cure, but various treatments can help to control the symptoms. Typically, topical agents are used for mild disease, phototherapy for moderate disease, and systemic immunosupressent agents for severe disease.

Key Terms

  • psoriasis: A noncontagious disease whose main symptom is gray or silvery flaky patches on the skin which are red and inflamed underneath when scratched.
  • plaque: A plaque is a raised area of inflamed skin covered with silvery white scaly skin and connective tissue.
  • autoimmune disease: A disease where one’s immune system attacks one’s own tissues.

Psoriasis is an autoimmune disease that affects the skin. It occurs when the immune system mistakes the skin cells as a pathogen, and sends out faulty signals that speed up the growth cycle of skin cells. Psoriasis is not contagious. The most common form, plaque psoriasis, is commonly seen as red and white colored scaly patches appearing on the top first layer of the skin. Plaques frequently occur on the skin of the elbows and knees, but can affect any area, including the back, scalp, palms of hands and soles of feet, and genitals. In contrast to eczema, psoriasis is more likely to be found on the skin of outer side of the joint rather than in the crease.

Symptoms and Complications

Depending on the severity and location of outbreaks, individuals may experience significant physical discomfort and some disability. Itching and pain can interfere with basic functions such as self-care, walking, and sleep. Plaques on hands and feet can prevent individuals from working at certain occupations, playing some sports, and caring for family members or a home. Quality of life is an issue for psoriasis patients, as they may feel self-conscious about the appearance of their skin and have a poor self-image that stems from fear of public rejection and psychosexual concerns. Psychological distress can lead to significant depression and social isolation.

Treatment

Psoriasis is typically a lifelong condition. There is currently no cure, but various treatments can help to control the symptoms. Typically, topical agents are used for mild disease, phototherapy for moderate disease, and systemic immunosuppressant agents for severe disease.

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Psoriasis: A person whose back and arms are affected by psoriasis.