Anaphylactic Shock
Anaphylactic response, allergic reaction, anaphylactic shock, sensitisation and hypersensitivity are commonly used as one of the possible consequences. Few still know how it develops or is manifested and fewer still know what they really mean despite using them like they really know.
First, some simple definition as they are commonly applied:
Allergy - state of hypersensitivity induced by exposure to a specific antigen resulting in harmful immunological reaction on subsequent exposures.
The key words are underlined.
Induced -Allergy is induced not caused which is to say it may not develop everytime
due to that agent in all people
Specific antigen -An antigen can be any substance but usually refers to exogenous
(foreign) substance which can induce a immune response. The same antigen must be used to
cause the same allergic reaction in sensitised people.
Harmful - All immuno-competent people like us mount an immune response to a foreign
antigen be it toxin or enzyme or just simple protein. There usually is some damage by
the immune system to our body. But to qualify as allergy, the immune response must be
sufficiently harmful as to outweigh the benefits of immunity and detoxification (in case
of toxins).
Subsequent - Allergy ONLY occurs after you have been exposed more than one time. In
this case, those stung for the first time by scorpions should rightfully not shows
allergic reaction but may be sensitised after that. However, similar substance may be
found in other arthropod stings or bites and if you have been bitten or stung by them,
allergic reaction is still possible.
Hypersensitivity - state of altered reactivity in which the body reacts with an exaggerated immune response to a foreign substance. It is divided into 4 types but is too complex to be discussed here.
Key words:
Altered - simply 'changed' :)
exaggerated - more than what is considered as normal
foreign - as mentioned in antigen. Foreign can be literally foreign or what the
body mistake to be foreign like in systemic lupus erythematous.
Sensitisation - 1) Administration of antigen to induce
a primary immune response. 2) exposure to allergen that results in
development of hypersensitivity.
In our context meaning 2) is mostly what is intended.
Key words:
Primary immune resposnse - main antibody (Ab): Immunoglobin M (IgM) mediated.
Basically exposed to it the first time.
Allergen - an antigen which causes allergy in sensitised people. Refer above for
the definitions.
Anaphylaxis (I simplify and rephrase
it)-
1) exposure of sensitised individual to a specific
antigen or hapten resulting in manifestation of immediate
hypersensitivity reaction usually resulting in urticaria, pruritus, angioedema followed
by vascular collapse and shock and often accompanied by life threatening respiratory
distress.
2) a general term originally applied to the situation in which exposure to toxin resulted
in development of hypersensitivity instead of
immunity.
3) Anaphylaxis has now been subsumed under the more general concept of immediate (Type I) hypersensitivity.
The REAL topic of this page. All definition one, two and three are applicable. Definition one usually is referred to by clinician as the symptoms. Definition two more related to sensitisation due to toxins and definition three is what actually is going on in the reaction.
Key words:
Specific antigen - refer to above
manifestation - fanciful word for 'showing signs of '
Immediate hypersensitivity - equated to Type I to be dealt with later.
urticaria - vascular reaction. shows local swelling (wheals) due to excessive fluid
collected (oedema) in the upper layer of skin (dermis). This is because of widening
(dilatation) and increased porosity (permeability) of small blood vessels (capillaries).
pruritus - itchiness of the skin.
angioedema - similar mechanism to urticaria. Gives rise to huge swelling and arises
from the deeper skin (dermis).
shock - 1) sudden disturbance in mental equilibrium 2)
condition of profound (very great) hemodynamic(blood circulation) or metabolic disturbance
characterised by failure of circulatory system to maintain adequate perfusion (blood
carrying oxygen) of vital organs (like liver, kidney, heart and brain). There are many
types and anaphylactic shock is one of them.
To most academic discussion, the second meaning is more applicable. To the victim, mental
shock may also set in thereby fulfiling both definitions.
Immediate Hypersensitivity (Type I) (IgE mediated)
Prior to this hypersensitivity, Immunoglobin M (IgM) which mediates primary immune response causes the immune cells (T-lymphocyte) to produce Immunoglobin E (IgE) instead of IgG in normal cases.
Simplified sequence of events-
Allergen like toxin in sensitised individual binds to 2 IgEs on the mast cells (another immune cell)
Mast cell when bound degranulates, releasing its content which includes histamine, heparin, proteolytic enzymes and chemotactic agents.
Histamine increases capillary permeability and dilates blood vessels causing oedema. At
the same time, it causes itch and constriction of bronchi (airways to lung) which may
result in respiratory distress.
Heparin prevents blood coagulation
chemotactic agents attracts immune cells like neutrophils, eosinophils etc
Also newly formed substances called Slow Reacting Substances of Anaphylaxis (SRS-A) like leukotrienes (LTC, LTD, LTE) which is released somewhat later causes further vasodilation and bronchoconstriction
Prostaglandins (PGD) is also formed and this causes fever (pyrexia), further bronchial constriction and inflammation.
How anaphylaxis causes harm
The excessive influx of immune cells and antibody secretion damages neighbouring tissues.
The inflammation, vasodilation and oedema formation causes sudden drop in circulating fluid in the body. As a result many important organs are not sufficiently supplied with oxygenated blood resulting in organ failure. This state is also known as shock. Serious cases cause almost immediate death as even the heart and brain has insufficient oxygen. This is one of the major reason for death in anaphylactic shock.
The bronchial (airway) constriction by histamine, SRS-A, LTD and PGD increases the difficulty of breathing. This is further complicated by the excessive fluid in the lungs from inflammation (pulmonary oedema) and insufficient blood going to the lungs. Hence, this gives rise to respiratory distress which is another major killer in anaphylaxis.
A minor point is that it causes itch all over..... like hundreds of giant mosquito bites.
Swelling of tongue and excessive production of saliva may obstruct airway. This is very true and serious and can cause death (my friend went into anaphylaxis right before me so I know!)