Hypersensitivity reactions: Introduction, Classification and Types

Content

It may be defined as undesirable reactions produced by normal immune system which includes allergies and autoimmunity.

Hypersensitivity reactions are classified traditionally into immediate and delayed type.

Immediate hypersensitivity (B cell or Antibodies mediated).

Delayed type

T cell mediated.
Infection type.
Contact dermatitis type.
Hypersensitivity reactions are inflammatory reaction within the humoral or cell mediated immune system death proceeds for tissue damage or death.
They are localized reaction induced by non infectious antigen which are called allergens.

Immediate hypersensitivity	Delayed hypersensitivity
Appears and recedes rapidly	Appear slowly, lasts longer
Induced by antigen or haptens by any route	Antigen or haptens intradermally or with Freunds reagent or by skin contact.
Antibody mediated reaction: In this type of hypersensitivity circulating Antibodies are present and they are responsible for Reactions.	Cell mediated reaction: Here circulating Antibodies are absent and they are not responsible for the Reactions.
Passive transfer is possible with the serum	Can’t be able to transfer with the serum
Desensitization easy but short lived	Difficult but it is long lasting

Hypersensitivity reactions is of four types:

1. Hypersensitivity Type 1
2. Hypersensitivity Type 2
3. Hypersensitivity Type 3
4. Hypersensitivity Type 4

Hypersensitivity Type-1

Anaphylactic
IgE or reagin dependent.
Antibodies, cytotropic IgE antibodies are fixed on the surface of tissue cells (basophils and mast cell) in sensitized individual.
Clinical complex of Type 1 reaction includes potential system and anaphylaxis and localized responses like hey fever and asthma.

Hypersensitivity Type- 2

Cytotoxic or cell stimulating.
These type of hypersensitivity reactions is initiated by IgG antibodies that may be reacts either with cell surface or tissue antigens.
ADCC-antibody dependent cell mediated cytotoxic reaction.
Cell or tissue damage occurred within the presence of complement or mononuclear cells.
Type 2 reactions are intermediate between hypersensitivity and autoimmunity.

Hypersensitivity Type- 3

Immune complex or toxic complex diseases.
In this type of hypersensitivity reactions, antigen-antibody complexes are responsible for damages.
These may precipitate in a round small blood vessel, causing damage to secondary cells or on membranes, interfering with their functions.

Hypersensitivity Type 4 (Delayed type hypersensitivity)

Delayed or cell mediated hypersensitivity.
Some cytokines like MCF, MIC and interferon – Gamma, are released from sensitized TH-1 cells and meditate the reactions.
The release of cytokines makes accumulation of macrophages which produce lytic enzymes and causes the tissue damage along with neutralization of the poison or antigen.

Type of Reactions	Clinical syndrome	Time required for manifestation	Mediators
Type 1: IgE type	Anaphylaxis Atopy	Minutes	IgE, histamine and other agent
Type 2: cytolytic and cytotoxic	Antibody mediated damage, anemia, agranulocytosis etc.	Variable hours to days	IgG, IgM
Type 3: immune complex	arthus reaction serum sickness	Variable : hours to days	IgM, IgG,C and leucocytes
Type 4: delayed hypersensitivity	Tuberculin contact dermatitis	Hours to days	T- cells, lymphokines, macrophages.

1% – https://en.wikipedia.org/wiki/Hypersensitivity
1% – https://emedicine.medscape.com/article/136118-overview
1% – https://www.slideshare.net/MartaGerasymchuk/metod-med-fty-1st-semester-book-2017-module-1
1% – https://www.studyblue.com/notes/note/n/nurse-pathophysiology-study-guide-2012-13-pittiglio/deck/9711393
1% – https://quizlet.com/170287342/immuno-4-flash-cards/