Hypersensitivity Reactions: Types and Mechanisms, Lecture notes of Cell Biology

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Hypersensitivity Reactions

• Body-damaging reactions that occur during

the immune response are called

hypersensitivity reactions (HSR).

• ADR is divided into four groups according

to formation mechanisms.

• • Type I-TypeII and TypeIII HSR; humoral

immunity (antibodies) and develops in a

short time.

• • Type IV HSR; related to cellular immunity

(T lymphocytes) and develops longer

TYPE I HYPERSENSITIVITY

REACTIONS

• Cells and molecules involved in Type I HSR; IgE, mast cells, basophils
• Vasoactive agents, histamine, serotonin etc. which are released from the stimulated mast cells and basophils, etc. substances.
• Anaphylaxis, inhalation allergies (hay fever), food allergies, parasitic allergies, vaccine and drug allergies

TYPE II HYPERSENSITIVITY

REACTIONS

• Hemolytic type or cytotoxic type HSR
• Cells and molecules involved in Type II HSR; foreign erythrocytes, IgM and IgG
• It develops in blood transfusions, blood conflicts of newborns, some drugs and microorganisms,
• Blood group antigens (eg 11 in cattle and dogs, 7 in horses, 6 in sheep)
• Heterophile antigen; antigens found in various plants, bacteria and parasites and cross-reacting with erythrocyte antigens

TYPE III HYPERSENSITIVITY

REACTIONS

• Immune complex type HSR
• It occurs against microbial

antigens or against autoantigens,

• The most important result of

immune complex formation is

complement activation,

• Type III HSR is local or generalized.

TYPE III HYPERSENSITIVITY REACTIONS

• Antigen-antibody reaction
• Complement addition and activation to the antigen-antibody complex
• Release of chemotactic factors by complement
• Infiltration and inflammation of neutrophils by chemotactic factors
• Circulating immune complex formation-generalized reaction
• Immune complex formation in tissues (kidney)-local reaction

TYPE IV HYPERSENSITIVITY

REACTIONS

• In tuberculosis infection, susceptible (memory) Th cells remain in the body for many years. Regardless of the way the antigen enters the body again, susceptible Th cells are activated and type IV HSR begins.

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