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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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Hypersensitivity Type IV
delayed hypersensitivity p. 110
type IV hypersensitivity p. 111
type IV hypersensitivity reaction p. 111
as type IV hypersensitivity p. 111
graft-versus-host disease p. 117
contact dermatitis p. 485
Having a hypersensitivity means that someone’s immune system has reacted to something in such a way that it ends up damaging them, as opposed to protecting them.
There are four different types of hypersensitivities, and in the fourth type or type 4, the reactions are caused by T lymphocytes, or T cells, and so type IV is also sometimes known as T-cell-mediated hypersensitivity.
T cells are called T cells because they mature in the thymus.
The two types of T cells that cause damage to tissues in type IV hypersensitivity are CD8+ T cells also known as killer T cells or cytotoxic T cells, as well as CD4+ T cells also known as helper T cells.
CD8+ killer T cells do exactly what their name implies - they kill things.
They are like silent assassins of the immune system that go after very specific targets.
In contrast, CD4+ T cells locally release cytokines, which are small proteins that can stimulate or inhibit other cells.
So CD4+ T cells act like little army generals coordinating immune cells around them.
But both CD8+ and CD4+ cells start off as naive cells because their T cell receptor or TCR has not yet bound to their target antigen - which is that specific molecule it can bind to.
Alright so let’s play out a scenario. Let’s say someone’s skin brushes up against poison ivy, and gets the molecule urushiol all over.
Type IV hypersensitivity is a type of delayed-type immune response, in which the immune system responds to an antigen several hours or days after exposure. It is also known as cell-mediated hypersensitivity because tissue damage involves T cells. This can be via either CD4+ T helper cells, which help coordinate the attack, or CD8+ killer or cytotoxic T cells, which directly destroy host cells. Examples include contact dermatitis, poison ivy, tuberculin skin test, and certain drug reactions, such as allopurinol. Treatment options for Type IV hypersensitivity may include medications like corticosteroids and avoiding exposure to the triggering antigen.
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