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What are type 1 allergies? Where are they most common and why? What are some examples?

An inappropriate response, by genetically predisposed hosts, to environmental antigens. This is caused by IgE, which normally acts against parasites. Allergies are particularly common in developed countries due to a low amount of parasites means overreactive IgE.
Some examples of this type are allergic rhinitis (hayfever), asthma (chronic lung disease), eczema.urticaria (skin disorders) or even anaphylaxis (acutely fatal).


What are allergies usually caused by?

Traditionally non infectious environmental allergens (e.g pollen, nuts, saliva and faeces of dust mites, milk, eggs, fish or even drugs like penicillin). These allergens may be released from larger particles. The pathway followed is the same as for antigens (dendritic cells taking to T cells).


How are mast cells involved in immune reactions? Why does this mean the allergic reaction doesn't strike the first time?

The IgE antibodies produced in the initial exposure bind to receptors on the mast cells. On subsequent exposure to the same allergen these attached IgE molecules bind to the antigen and lead to degranulation of the mast cell(due to cross linking of adjacent IgE molecule), this causes release of histamine and other chemicals, leading to the allergy symptoms.

There is no allergic reaction at first contact as there hasn't been time to form the IgE necessary to cause the response before the allergen is removed from the system.


What allergy affects occur in the gastrointestinal tract, airways and blood vessels and what do they lead to?

Gastrointestinal tract: increased fluid secretion and increase peristalsis (a series of wave-like muscle contractions that moves food to different processing stations in the digestive tract.) This leads to expulsion of it's contents (diarrhea and vomiting).

Airways: Decreased diameter and increased mucus
secretion leading to congestion and blockage of airways (wheezing, coughing, phlegm and asthma), also leads to swelling and mucus secretion in nasal passages.

Blood vessels: increased blood flow and increased permeability leading to increased fluid in tissues which increases flow of lymph in lymph nodes, increases cells and protein in the tissues and increases effector response in tissues.


Who is most at risk of developing allergies? How can allergies be gotten rid of?

Children are most at risk. Allergies can be stopped by minute amounts of the allergen which increases with each immunotherapy session until once all the IgE is gone a large amount is given. This causes IgG to be produced permanently instead, which doesn't cause the allergic reaction.


How can we diagnose allergies?

We can test for allergies via blood samples (if the mast cells degranulate you are allergic to the tested substance) or we can do a wheal and flare reaction where we inject a minute amount of common allergens into the back of the individual (flared up areas are things the individual is allergic to). We use a histamine control group to see what allergic sites will look like and a saline control group to ensure the reaction isn't to the needle or something (should show no reaction).


What is a type II allergy? Give examples

In type II hypersensitivity (or cytotoxic hypersensitivity) the antibodies produced by the immune response bind to antigens on the patient's own cell surfaces. The antigens recognized in this way may either be intrinsic ("self" antigen) or extrinsic (absorbed onto the cells during exposure to some foreign antigen). These cells are recognized by macrophages or dendritic cells, which act as antigen-presenting cells. This causes a B cell response, wherein antibodies are produced against the foreign antigen.
Examples of this are haemolytic anaemia which causes red blood cells to be destroyed leading to only reticulocytes (new red blood cells) and spherocytes (dead blood cells), this can also develop against specific tissues e.g Goodpasture's syndrome which is antibodys forming against the glomerulus/kidney).


What is a type III allergy? Give examples

This is the immune complex disease, caused by antigen complexes and antibodies existing at the same time. These end up in our joints and kidneys, leading to symptoms like rash, headache, sore joints, kidney disease. Examples of this are systemic lupus erthematosus or rheumatoid arthritis.


What is a type IV allergy? Give examples

Delayed type hypersensivity, involves T helper cells and macrophages (chronic pathogen response). The overusage of this pathway leads to a reaction which takes a lot of time to show (24 hours). Typically this occurs when a disease like tuberculosis is injected into the skin of an individual whom already has the disease due to the amount of macrophages in the area (macrophages are harmful to surrounding cells). Leprosy or chemical allergies like dermatiis can also be an example of this due to chemicals binding to self proteins is also an example of this.


What is a type V allergy? Give examples.

This is caused by antibodies (IgG specifically)stimulating other cells e.g graves disease, this stops the negative feedback loop of the thyroid gland. Babies born from mothers with this type of hypersensitivity will have symptoms for roughly 4 months (at which point the antibodies wear off).


What is immunodeficiency?

Anything which decrease the immune systems ability to function, these are things like war, HIV, stress or genetics.


What happens if there are problems with epithelial cells in the thymus?

T cell production problems will occur, leading to no helper T cells and hence no B cells either.


What is diabetes mellitus type I caused by?

Cytotoxic T lymphocytes which act against the beta cells in the pancreas, leading to insulin production stopping completely.