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Type I hypersensitivity
Autoimmune hemolytic anemia
Hemolytic disease of the newborn
Rheumatic heart disease
Type II hypersensitivity
Systemic lupus erythematosus
Type III hypersensitivity
Type IV hypersensitivity
Common variable immunodeficiency
Hyperimmunoglobulin E syndrome
IgG subclass deficiency
Isolated primary immunoglobulin M deficiency
Selective immunoglobulin A deficiency
Adenosine deaminase deficiency
Hyper IgM syndrome
Severe combined immunodeficiency
Cytomegalovirus infection after transplant (NORD)
Post-transplant lymphoproliferative disorders (NORD)
Chronic granulomatous disease
Leukocyte adhesion deficiency
Blood transfusion reactions and transplant rejection: Pathology review
Immunodeficiencies: Combined T-cell and B-cell disorders: Pathology review
Immunodeficiencies: Phagocyte and complement dysfunction: Pathology review
Immunodeficiencies: T-cell and B-cell disorders: Pathology review
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So let’s start with the proteins that make up the classical pathway - C1, C2, C3, C4, C5, C6, C7, C8, and C9.
And parts of these proteins are designated with lower case letters like “a” or “b”. Pretty easy right?
Now these were numbered, in the order they were discovered, but not the order in which they function.
Generally speaking, each complement protein is normally inactive, and it becomes activated when it’s cleaved - in other words when some part of it breaks free.
A bit like how a fire extinguisher isn’t “active” until a pin is pulled out.
The complement system helps with three important immune processes: inflammation, phagocytosis, and the creation of membrane attack complexes or MACs.
Inflammation is when chemicals and cells collect to protect a damaged or infected area; phagocytosis is when certain white blood cells called phagocytes engulf and digest antigens; and membrane attack complexes are structures which dig into antigen surfaces and lyse them, or rupture and kill them.
There are actually three complement pathways: The classical pathway - called that because it was discovered first, the alternative pathway which was found second and is always at work, and the Lectin binding pathway - which was found third and when folks got more descriptive with their naming.
So all three pathways start out a bit differently, but end the same way - with a membrane attack complex which is a protein complex that creates a hole in a bacterial cell membrane - effectively destroying mainly gram negative bacteria.
The various complement fragments contribute to these three pathways, but also play other additional roles as well.
For example, C3b serves as an opsonin - which means that it helps immune cells phagocytose debris.
A complement deficiency is a condition in which the body doesn't have enough of the proteins called complements, which work together to fight infection. Complement deficiencies can be inherited or acquired. Inherited complement deficiencies are caused by a change (mutation) in the gene that codes for one of the proteins in the complement system. Acquired complement deficiencies can be caused by infections, cancer, and autoimmune diseases such as lupus. People with an inherited or acquired complement deficiency are at risk of severe infections, including meningitis, sepsis, and pneumonia.
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