Hypersensitivities

Hypersensitivities

When the immune system damages tissues while fighting off threats, such as animal dander or pollen, hypersensitivities develop. Hypersensitivities are clas-sified by how long they persist and if they involve T cells or antibodies. Delayed hypersensitivities are related to T-cell activity. Immediate and subacute hypersensitivities are caused by antibody-associatedreactions. Immediate hypersensitivities are also called acute or type I hypersensitivities. Commonly, peoplerefer to this type as allergies.

Type I Hypersensitivities

When an immune response occurs because of a non-harmful substance, it is called an allergic response. Immune and allergic responses sensitize the lym-phocytes, and the antibodies produced may combine with antigens. Allergic reactions can damage tissues, however, whereas normal immune responses cannot. Allergens are a type of antigens that trigger allergicresponses, which begin within seconds after contact.

A fairly rare type I hypersensitivity is anaphylac-tic shock, which is systemic. It is usually caused by anallergen directly entering the blood and circulating quickly through the body. The most frequent causes of anaphylactic shock include injection of foreign substances such as penicillins, which act as haptens; bee stings; and spider bites. Prompt administration of antihistamines can block actions of histamine and prevent many signs and symptoms of immediate ­hypersensitivity. Popular antihistamines include the over-the-counter drug diphenhydramine hydrochloride(Benadryl®).Severe anaphylaxis is treated with inject-able antihistamines, epinephrine, and corticosteroids.

Type II Hypersensitivities

Both the type II and type III hypersensitivities are classified­ as “subacute.” They are also caused by ­antibodies but not the same one as in the type I hyper-sensitivities. They have a slower onset and a longer reac-tion. They usually occur within one to three hours after antigen exposure and last for between 10 and 15 hours.

Type II hypersensitivities are related to IgG and are also called cytotoxic reactions. They occur when antibodies bind to antigens on certain body cells. Cytotoxic reactions stimulate phagocytosis and ­complement-mediated lysis of cellular antigens. These reactions may occur after transfusion of blood that was not matched correctly, which results in comple-ment lysing the transfused blood cells.

Type III Hypersensitivities

Type III hypersensitivities are known as immune-­complex hypersensitivities. They are related to IgMand occur when antigens are distributed throughout the body or blood, with the many formed insoluble ­antigen–antibody complexes unable to be cleared from a certain area. Often, this is linked to an autoimmune disease or a persistent infection. Intense inflammation results, including complement-­mediated cell lysis and death via the actions of neutrophils. This severely damages local tissues, which is known as necrosis. Examples of immune-complex hypersensitivities include glomerulonephritis caused by systemic lupus erythematosus and farmer’s lung caused by inhalation of moldy hay.

Type IV Hypersensitivities

Type IV hypersensitivities are also called delayedhypersensitivities and are linked to T cells. Theydevelop over one to three days, with inflammation and tissue damage occurring because of cytokine-ac-tivated macrophages or cytotoxic T cells. Examples include allergic contact dermatitis , often caused by contact with poison ivy, nickel and other metals, cos-metics, and deodorants. Also, type IV hypersensitiv-ities are used when testing the skin for tuberculosis. After introduction of tubercle antigens, a person who has been sensitized to these antigens develops a small, hard lesion that lasts for days. This constitutes a posi-tive result, and tuberculosis is diagnosed.

Tissue Rejection Reaction

Another type of reaction concerns transplantation and tissue rejection. When a body part is transplanted from one person to another, the receiving patient’s immune system may recognize the transplanted part as foreign and attempt to destroy its tissues, causing tissue rejection reaction. A tissue rejection is simi-lar to the cellular immune response against foreign antigens. The greater the difference between the antigens on cell surface molecules of the donor and recipient, the greater and more rapid the rejection reaction. Therefore, donor and recipient tissues must be matched to minimize these reactions. A donor’s tissues must be antigenically similar to the person receiving the transplant.

The four major types of transplant tissue, or grafts, include the following:

■■ Isograft from an identical twin (example: bone marrow from a healthy twin to a twin who has leukemia)

■■ Autograft from another part of the recipient’s own body – this is not an actual “transplant” since it comes from the same individual (example: skin from one area of the body to replace burned skin in a different area)

■■ Allograft from another person who is not an identical twin (example: kidney transplant from a relative or a closely-matched donor)

■■ Xenograft from another species, such as baboonsor pigs (example: heart valves from an animal)

Immunosuppressive drugs are used to reduce tissue rejection. Although they reduce the immune response by suppressing antibody and T-cell formation, they weaken the recipient’s immune system. Frequently, transplant patients survive the transplant but die from an infection caused by a weakened immune system.

1. Define the term “autoimmune disease” and list examples.

2. Explain the type of hypersensitivity that is also known as an allergic response.

3. When glomerulonephritis develops as a result of systemic lupus erythematosus, which type of hypersensitivity is related?