Table of Contents
Under normal human physiological conditions, complement proteins circulate in the blood in an inactive form. When activated by a foreign body, they become enzymatically active and trigger several molecules in a series of reactions. They perform a number of functions, including:
- Promoting inflammation
- Recruiting immune cells
- Killing foreign bodies.
Complement reactions occur in two distinct pathways: the classical and the alternative pathway.
Classical pathway: an immune complex binds to C1, which then cleaves proteins C2 and C4. This causes cleavage of C3 and then of C5,6,8,9. A membrane attack complex forms, and cell lysis occurs.
Alternative pathway: Bacterial cell wall activates C3, causing its cleavage and then cleavage of C5,6,8,9. A membrane attack complex forms, and cell lysis occurs.
During a disease, complement systems can either be overactive or underactive. Complement deficiencies of the molecules that inhibit the system will cause an overactive response. Deficiencies in proteins that activate the system (like C3) lead to underactive responses. This means that patients are more susceptible to bacterial and viral infections.
Deficiencies in decay-accelerating factor (DAF)
In normal human physiology, this protein will inhibit C3 and C5 convertase, stopping membrane attack complex formation and the attack on human cells. Without it, red blood cells, in particular, are much more susceptible to lysis, and patients present with paroxysmal nocturnal haemoglobinuria.
Deficiencies in C1 esterase inhibitor (hereditary angioedema)
Deficiency in C1 esterase inhibitor will result in an overactivity of C1. This causes excess production of anaphylatoxins, which leads to swelling of the dermis, subcutaneous tissue, mucosa and submucosal tissues, i.e., angioedema. C1 esterase inhibitor deficiency is known as hereditary angioedema. It is an autosomal dominant hereditary condition that occurs in 1 in 10,000 individuals. Individuals with C2 deficiency have an increased risk of infection.
Deficiencies in C2 and C4
Loss of the complement system by deficiencies in C2 and C4 results in underactivity and lack of immune activity. These conditions can present similarly to autoimmune disorders like systemic Lupus erythematosus. Some C2 deficiencies have been associated with autoimmune diseases like Lupus.
C3 deficiencies are associated with a number of conditions. Reduced levels of C3b increase the probability of developing infections with severe pyogenesis. This is caused by reduced opsonization. Individuals with C3 deficiencies are also more susceptible to type III hypersensitivity reactions (reduced clearance of antigen-antibody-C3b complexes from the circulation causes increased risk of hypersensitivity reactions).
As noted above, C5-9 form the membrane attack complex that causes cell lysis. Without this, gram negative bacteria are far more likely to cause a serious infection. Of note, patients with C5-8 deficiencies are particularly susceptible to Neisseria infections.
Phagocytic Cell Disorders
Phagocytic cell disorders interrupt the development or functioning of phagocytes, which under normal physiological conditions ingest and digest pathogens as part of a humoral immune response.
Chronic granulomatous disease
In normal human physiology, phagocytes (such as neutrophils) phagocytose (or ingest) bacteria before breaking them down via reactive oxygen species. To form these reactive oxygen species, an “oxidative” burst is precipitated by enzymes such as NAPDH oxidase.
Polymorphisms in the gene that codes for NADPH oxidase can result in chronic granulomatous disease. In CGD, the oxidative burst does not occur, and phagocytes are unable to break down invading pathogens. There are several types of CGD, and around 1 in 200,000 individuals are diagnosed with the disease.
An inability to break down pathogens causes an increased susceptibility to a number of bacterial and fungal infections.
This is a rare autosomal recessive disorder caused by defective neutrophil lysosome function. It is characterized by recurrent streptococcal and staphylococcal infections.
T-helper cells are unable to produce IFN-Ɣ. This reduces macrophage activation. TH2 pathway is increased and histamine is released. Increased histamine causes neutrophil chemotaxis, and the patient may suffer from recurrent “cold.” The syndrome can also result in atopic dermatitis and staphylococcal abscesses.
This autosomal dominant condition causes irregular production of G-CSF (which stimulates the production of granulocytes). Usually, patients will have a neutropenia for 3 days every 21 day cycle. During this period, they are susceptible to bacterial infections.
Leukocyte adhesion deficiencies
Polymorphisms in integrins on leukocytes cause improper neutrophil migration and recurrent infections. It is also important to note that this syndrome can cause delayed umbilical cord separation.
Both IL-12 and IFN-Ɣ receptor can be defective. In IL-2 receptor deficiency, TH1 responses are suppressed. In IFN-Ɣ receptor deficiencies, macrophage activation is inhibited. Both lead to mycobacterial infections.
This syndrome can clinically resemble cystic fibrosis. It is often characterised by pyogenic infections and exocrine pancreatic insufficiency. Neutropenia is caused by a defect in neutrophil migration.
Autoinflammatory disorders are not to be confused with autoimmune disorders. In autoinflammatory disorders, the innate immune system causes inflammation without a known input. Autoinflammatory disorders will typically present with fever, rash etc. (signs of inflammation).
Familial Mediterranean fever
This disease typically results in 1-3 day episodes of fever. Mutations on the gene encoding IL-1β cause the disease. It presents in childhood in patients of Jewish, Arab or Turkish descent (among others).
This typically presents in adults with oral and genital ulcers. It is common in the Middle East and Asia. The disease causes inflammation of blood vessels. However, the genetic underpinnings of the disease are currently unknown.
Neonatal onset multisystem inflammatory disease
This disorder often develops in the first few weeks of life and causes inflammation in multiple organ systems, including the nervous system, skin, etc. Newborns can develop fever and meningitis. Mutations in NLRP3 account for around 60% of patients with the disease. However, the underlying mechanism that causes the inflammation is currently unknown.
Mutations Affecting T-Cell Maturation
Mutations affecting T-cell maturation can often lead to autoimmune conditions whereby the immune system attacks self-antigens. Mutations can occur in a number of different areas including the thymus during T-cell selection and in regulatory T-lymphocytes.
Typically, during T-cell maturation, TCR’s (T-cell receptors) with high affinity for self-antibodies are deleted in the thymus. The thymus expresses self-antigens in order to select TCRs with high affinity. Therefore, mutations in genes causing expression of self-antigens in the thymus can lead to autoimmune conditions.
Mutations in the AIRE gene (this gene is involved in the expression of thymic self-antigens) can lead to autoimmune polyglandular syndrome – whereby patients develop a number of autoimmune diseases.
In some patients, mutations can occur in regulatory T-lymphocytes. For instance, mutations in the Foxp3 gene (a transcription factor for T-regulatory lymphocytes) can cause IPEX (Immune dysregulation, Polyendocrinopathy, Enteropathy, X linked syndrome). Patients develop a number of autoimmune conditions after birth, including type 1 diabetes.
Common variable immunodeficiency
This type of immunodeficiency characteristically presents in early adulthood with recurrent upper and lower respiratory tract infections. IgG levels are typically below 0.5 g/L. The disease is caused by a myriad of genetic mutations and typically affects 1 in 50,000.
This heritable disease is caused by a lack of B cells. B-cell development falters at the pre-B stage. An X-chromosome gene polymorphism for tyrosine kinase causes a loss of function mutation that inhibits B-cell maturation.
The disorder will present shortly after birth, when maternal immune (IgG) protection falls. Presentations with recurrent respiratory tract infections are characteristic. Treatment is usually given as IV immunoglobulins. Prognosis was historically poor, but recently, patients have survived into adulthood. Chronic lung disease and lymphomas are complications that can cause mortality.
Selective IgA deficiency
This is a relatively common primary immunodeficiency, occurring in 1 in 600 people in some parts of Europe. Recurrent infections typically cause presentation. There is no known pathogenesis.
Hyper IgM syndrome
X-linked mutation in the CD40 ligand gene causes abnormal signalling between B- and T-lymphocytes. Typically, CD40 is involved in the generation of B-lymphocytes with high affinity immunoglobulin. It is also involved in the maturation of T-cells. The disease is mild and often only presents in later life. Opportunistic infections will occur.
This is an X-linked condition causing cytoskeletal functional deficits. Eczema and thrombocytopenia are observed.
MHC class II deficiency
This deficiency is often referred to as bare lymphocyte syndrome. It is a rare immunodeficiency; it accounts for a small (5%) portion of severe combined immunodeficiency. The syndrome causes lack of HLA class II expression. This results in a lack of T-cell immune response. Patients are highly susceptible to infection.
Severe Combined Immunodeficiency (SCID)
SCID is an umbrella term to describe a number of disorders resulting in impaired B- and T-cell activity. It affects just 1 in 100,000 individuals. The most common mutation is found on the IL-2 receptor gene.
Typically, newborns will suffer intracellular pathogenic infections, as humoral immunity is received by IgG and IgA in breast milk. Newborns suffer from opportunistic infections, and without treatment the disease is fatal. Treatment is typically one of the following:
- Stem cell transplantation
- Gene therapy
- Enzyme replacement therapy.