THE ADAPTIVE IMMUNE SYSTEM
B. Primary Immunodeficiency
The overall purpose of this Learning Object is:
1) to learn the basics about four types of conventional primary immunodeficiencies (B-lymphocyte disorders, T-lymphocyte disorders, combined B- and T-lymphocyte disorders, and innate immunity disorders; and
2) to learn what is meant by novel primary immunodeficiencies.
LEARNING OBJECTIVES FOR THIS SECTION
Adaptive (acquired) immunity refers to antigen-specific defense mechanisms that take several days to become protective and are designed to remove a specific antigen (def). This is the immunity one develops throughout life. There are two major branches of the adaptive immune responses: humoral immunity and cell-mediated immunity.
1. humoral immunity (def): humoral immunity involves the production of antibody molecules in response to an antigen (def) and is mediated by B-lymphocytes.
2. cell-mediated immunity (def): Cell-mediated immunity involves the production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells, and cytokines in response to an antigen (def) and is mediated by T-lymphocytes.
Immunodeficiency results in an inability to combat certain diseases and may be of two types: primary or secondary. Primary immunodeficiency (def) is usually an immunodeficiency that one is born with. In the case of secondary immunodeficiency (def), one is born with normal immune responses but some secondary factor or occurrence causes a decrease in immune responses.
In this section we will look at primary immunodeficiencies.
B. Primary Immunodeficiency (def)
A primary immunodeficiency is usually an immunodeficiency that one is born with. Until recently, primary immunodeficiencies were defined as a rare recessive genetic defect in the immune responses that involved the development of B-lymphocytes, T-lymphocytes, or both and resulted in multiple, recurrent infections during infancy. Depending on the disorder, the lymphocytes in question were either completely absent, present in very low levels, or present but not functioning normally. These disorders represent the conventional immunodeficiencies.
However, based on our increased understanding of the human genome and immune responses it now appears that there are a multitude of common, less severe primary immunodeficiencies involving just one or more of the huge number of genes involved in the immune responses. These so called novel primary immunodeficiencies involve the decreased ability to combat just a single type of infection or a narrow range of infections.
1. Conventional Immunodeficiencies (def)
The conventional primary immunodeficiencies were grouped as follows:
a. B-lymphocyte (def) Disorders
There may be greatly decreased humoral immunity (def) but cell-mediated immunity (def), mediated by T-lymphocytes, remains normal.
1. Agammaglobulinemias (def)
Few if any antibodies are produced and there are reduced B-lymphocyte numbers. The person is very susceptible to recurrent infections by common pyogenic bacteria such as Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Neisseria meningitidis, and Hemophilus influenzae.These bacteria have antiphagocytic capsules that are normally eliminated by antibodies through opsonization.
2. Hypogammaglobulinemias (def)/Isotype Defects (def)
Decreased general antibody production or decrease production of a single isotype of antibody. Examples include:
- IgG2 subclass deficiency. A person is unable to produce the subclass of IgG called IgG2 but can produce other classes of antibodies. There is increased susceptibility to bacterial infections.
- Selective IgA deficiency. A person is unable to make IgA but can produce other classes of antibodies. There is increased susceptibility to bacterial infections and certain prozozoan infections.
- Combined Variable Immunodeficiency (CVID). Hypogammaglobulinemia with normal or decreased numbers of B-lymphocytes.
More severe forms such as agammaglubulinemia are treated with artificially-acquired passive immunization (def) - periodic injections of large amounts of immune globulin (IG or IVIG).
b. T-lymphocyte Disorders
There is little or no cell-mediated immunity (def) if the disorder involves T8-lymphocytes (def) and/or T4-helper lymphocytes (def). There may also be decreased humoral immunity (def) if there is a disorder involves T4-helper lymphocytes.
1. MHC Expression Defects
- MHC-I (def) deficiency. Decreased levels of MHC-I production and reduced T8-lymphocyte numbers.
- Bare lymphocyte syndrome. Decreased levels of MHC-II (def), decreased numbers of T4-lymphocytes, and decreased T4-dependent antibody production (def) by B-lymphocytes.
2. T-Lymphocyte Signaling Defects
- Wiskott-Aldrich syndrome. Defective T-lymphocyte activation and defective leukocyte mobility.
- Proximal TCR (def) signaling defects. Defective cell-mediated immunity and defective T4-dependent antibody production by B-lymphocytes.
3. Familial Hemophagocytic Lymphohistiocytosis
- Perforin (def) deficiencies. Defective CTL (def) and NK cell (def) function; uncontrolled activation of macrophages (def) and CTLs.
- Granule fusion defects. Defective CTL and NK cell function; uncontrolled activation of macrophages and CTLs.
- X-linked lymphoproliferative syndrome. Defective CTL and NK cell function; uncontrolled activation of macrophages and CTLs. Uncontrolled Epstein-Barr virus - induced B-lymphocyte proliferation.
c. Combined B- and T-lymphocyte Disorders (Severe Combined Immunodeficiency Disease or SCID)
These deficiencies affect both humoral immunity (def) and cell-mediated immunity (def). There is a defect in both B-lymphocytes and T-lymphocytes, or just T-lymphocytes in which case the humoral deficiency is due to the lack of T4-helper lymphocytes.
1. Cytokine-Signaling Defects
- Autosomal recessive SCID. Shows a marked decrease in T-lymphocytes but normal to increased levels of B-lymphocytes. There is reduced antibody levels due to the lack of T4-helper lymphocytes.
- X-linked recessive SCID. Shows a marked decrease in T-lymphocytes but normal to increased levels of B-lymphocytes. There is reduced antibody levels due to the lack of T4-helper lymphocytes.
2. Defects in Nucleotide Salvage Pathways (def)
- PNP deficiency. Shows a progressive decrease in both T-lymphocytes, B-lymphocytes, and NK cells, as well as reduced antibody levels.
- ADA deficiency. Shows a progressive decrease in both T-lymphocytes, B-lymphocytes, and NK cells, as well as reduced antibody levels.
- RAG1 or RAG2 deficiency. Shows ab absence or deficiency of both T-lymphocytes and B-lymphocytes, as well as reduced antibody levels.
- ARTEMIS defects. Shows ab absence or deficiency of both T-lymphocytes and B-lymphocytes, as well as reduced antibody levels.
4. Defective Thymus Development
The thymus is needed for the development of T-lymphocytes from stem cells.
- DiGeorge syndrome. Shows decreased levels of T-lymphocytes, normal levels of B-lymphocytes, and reduced antibody levels.
- Defective pre-TCR checkpoint. Shows decreased levels of T-lymphocytes, normal or reduced levels of B-lymphocytes, and reduced antibody levels.
d. Innate Immunity Disorders
- Chronic granulomatous disease. No oxygen-dependant killing pathway in phagocytes. Recurrent intracellular bacterial and fungal infections.
- Leukocyte adhesion deficiencies. Defective leukocyte adhesion, diapedesis (def), and migration. Recurrent bacterial and fungal infections.
- Chediak-Higashi syndrome. Defective vesicle fusion and lysosomal function in neutrophils, dendritic cells, macrophages and other cells. Recurrent infections by pyogenic bacteria.
2. Novel Immunodeficiencies (def)
While the rare conventional primary immunodeficiencies mentioned above are still very important, based on our increased understanding of the human genome and immune responses it now appears that there are a multitude of common, less severe primary immunodeficiencies.
These so called novel primary immunodeficiencies relate to an individual’s own unique genetics and can involve one or more of many immunity genes, ranging from any of the huge number of genes conferring protective immunity in general, to individual genes conferring specific immunity to a single pathogen.
It is now thought that almost every person suffers from one form of primary immunodeficiency or another. Unlike the classical primary immunodeficiencies, however, these primary immunodeficiencies involve the decreased ability to combat just a single type of infection or a narrow range of infections. Examples include:
- Disorders of the interleukin-12/interferon-gamma pathway appear to make individuals more susceptible to Mycobacterium and Salmonella infections.
- Disorders of the TLR-3 pathway (def) makes individuals more susceptible to herpes simplex virus encephalitis.
- Disorders of the toll-interleukin 1 receptor/nuclear factor kappa B pathway makes individuals more susceptible to staphylococcal and pneumococcal infections.
- Disorders of properdin (def) and terminal components of the complement pathways (def) make individuals more susceptible to Neisseria infections.
- People with chronic sinusitis that does not respond well to treatment have decreased activity of TLR-9 and produce reduced levels of human beta-defensin 2, as well as mannan-binding lectin needed to initiate the lectin complement pathway.