The Allergic Reaction

The allergic reaction is a process of the immune system reacting to a harmless substance as harmful. The process consists of:

1. Identifying the foreign substance
2. Producing the correct kind of antibody to fight the non-self particle
3. Launching an attack

A foreign substance that we breathe in is called an antigen. If the antigen can cause an allergic reaction, it is known as an allergen. Common airborne allergens come from;

• Pet dander
• Dust mites
• Cockroach
• Mold
• Pollen

In order for the immune system to provide protection to the body, it must have the ability to distinguish between self and non-self substances. On the membrane of every cell are various proteins. Together, these proteins molecules are called MHC’s. MHC stands for Major Histocompatibility Complex. The cells in our body all have similar MHC’s. Cells, molecules, and substances that are foreign to our body will have a different MHC signature. This MHC signature is how our immune system determines what is self and what is foreign.

The process begins by the detection of the allergen. Upon first exposure, IgE antibodies are produced but no symptoms occur. When the exposure of the same specific allergen occurs again, the IgE antibody activates a response that results in common allergy symptoms.

The body has millions of cells that perform the function of detecting foreign substances. B cells and T cells do this job for the immune system but do so in different ways. As seen in the diagram below, T cells need the help of Antigen-Presenting Cells (APC’s).

There are several different kinds of APC’s. In the allergic reaction process, Dentritic cells play a big role. As part of their chemistry makeup, they have potent Th cell activators. They are present in the tissues that are ports of entry into the body such as the skin, inside tissues in the nose, and lung tissues. They grow branched projections (the dentrites) which give the cell its name. These cells are constantly surveying their surrounding areas looking for non-self markers.

Picture drawn by David Wright.

FIGURE 2T: Once a foreign object (pollen) is discovered, it activates the dentritic cell. The dentritic cells capture the cells that are displaying a foreign MHC signature on its surface with its pattern recognition receptor.

Figure 2T, 3T, and 4T shows the function of Antigen-Presenting Cells at the initial stages of the allergic reaction.

FIGURE 3T: Once the pollen is consumed, the antigen-presenting cell digests it in preparation of presenting it to another cell of the immune system. The pollen is broken down to peptides and attached to a Class II MHC protein.

FIGURE 4T: The fragment of the antigen has moved to the outside of the cell membrane and is being displayed on the surface using MHC molecules. The MHC molecules are being displays on the surface. It is presented to a T cell (a pre-Th cell) that recognizes the protein. As the T cell recognizes the Class II MHC molecule complex and interacts with it, the APC sends a signal to the Pre-Th cell to activate it.

The antigen, the MHC molecule and the signal sent from the antigen-presenting cell is going to determine if the Pre-Th cell turns into a Th1 or Th2 cell.Th1 cells: In the case of a virus where the attack will need to take place inside of cells, a Th1 cell will develop. Killer T cells will be produced to perform this function.Th2 cells: In the case of allergens, the antibodies produce will be sent into the blood. Helper T cells will be produced in an allergic reaction.

FIGURE 5T: In the case of Pollen (and other allergens that cause an allergic reaction), the signals that the pre Th cell has received influence it to become a Th2 helper T cell.

The enemy has been identified and now the function of the helper T cell is to find the correct B cell that can produce the correct antibody. Once the correct kind of B cell has been found, it is the job of the helper T cell to activate it.

Figure 2B through 5B shows the process of how the B cell gets ready to be activated from a helper T cell.

FIGURE 2B: B cells have recognition receptors that allow it to recognize foreign antigens. Unlike T cells, the B cells recognize whole pathogens without any need for antigen processing. The B cell captures the pollen.

FIGURE 3B: The B cell consumes and break the pollen down by digesting it.

FIGURE 4B: After breaking the pollen down, the B cell takes the unique MHC molecules and displays it on the surface. Figure 2B showed the B cell acting as an antigen recognition cell. Now it is acting as antigen-presenting cell. B cells display antigen fragments bound to its unique MHC molecule.

FIGURE 5B: The broken down pollen pieces (antigen peptides) in association with class II MHC markers will attract the attention of a helper T cell.

FIGURE 6: The B cell and helper T cell find each other because they have both been stimulated from the same antigen. The only difference has been that they were attracted to different identifying markers on the pollen. In both cases, the discovering of the pollen stimulated both the T cell and B cell. This coming together now activates both of them.

The processed antigen peptide is presented to the helper T cell in association with MHC Class II identifying markers. This activates the T cell. As a result, it will send a chemical message to the B cell which activates the B cell.

The chemical message sent to the B cell will instruct it to begin production of antibodies and will also instruct it on what kind of antibodies to produce. In the case of allergens, IgE antibodies will be produced.

The diagram below shows the second part of the allergic reaction.

Picture drawn by David Wright.

FIGURE 7: Once activated, the B cell will differentiate into two types of B cells.

1. Plasma Cells: The manufacture of the correct kind of antibody. They are short-lived and will fade away when there is no longer exposure to the allergen and their antibody production is no longer needed.
2. B Memory Cells: These are clones of the original. They have a long life span. The encounter with the allergen will forever be remembered. If they ever come into contact with the same kind of allergen again, they are capable of producing an even faster response.

B cells that come into contact with an antigen for the first time are called naïve B Cells. There are millions of different kinds of B cells standing watch to detect any possible non-self MHC molecules. Upon activation, the immune system will launch an attack.

FIGURE 8: The plasma cells begin immediately producing the correct kind of antibody which is based on the information that the help T cell passed on to the B cell. It is programmed to make only one specific kind. One kind of IgE antibody is made for mold, another kind of antibody is made for ragweed. The antibodies are released into the blood as soon as they are made.

FIGURE 9: The IgE antibodies released by the plasma cells attach themselves to basophil cells in the blood and to mast cells in the tissue. Both types of cells have special receptors for the IgE antibody. Mast cells are found in large quantities in the tissue lining of the nose, lungs, and eyes they are the most involved with the allergic reactions.

Inside the mast cell is a nucleus. Around the nucleus are granules. These granules consist of many different kinds of inflammatory chemicals. Histamine is the most well known chemical of the allergic reaction.

Figure 9 shows the condition when allergist refers to the patient as being sensitized. There has been an exposure to an allergen and the antibody to fight that specific allergen has been produced. The antibody waits for the next exposure as it is attached to the mast cells. The next exposure will produce the allergic reaction.

FIGURE 10: When there is another exposure to the allergen for which the antibody was made for (pollen in this example), IgE antibodies captures it. Instead of one antibody attaching to one pollen particle, two antibodies side by side attach onto the same allergen. This creates a distortion to the outside membrane of the mast cell resulting in a rupture of the mast cell.

FIGURE 11: The rupture of the mast cell causes the release of histamine and other inflammatory chemicals to flow and to burst out. The release is very quick. The release of the mast cell granules is called degranulation.

It is the chemicals from inside the mast cells that cause the symptoms from an allergic reaction. These symptoms may include:

• runny nose
• irritation
• itching
• sneezing
• nasal congestion
• excess mucus production
• swelling of tissues
• smooth muscle contraction
• dilatation of blood vessels
• fluid leaking from cells

When there is no longer exposure to the allergen, the chemicals and plasma cells will fade away. Only B memory cells will stay behind on watch for another exposure.

The reaction above will occur for all the different types of hay fever:

• Pollen Allergy
• Pet Allergy
• Mold Allergy
• Dust Mite Allergy

This misguided response of the immune system for people who have allergies is the over-production of IgE antibodies. Allergic rhinitis is an IgE mediated response to the allergen.

The unusually large amount of IgE antibodies in the blood system is one of the characteristics that distinguish between an allergic immune system and a person who does not have nasal allergies. The large quantity of IgE antibodies in the blood system makes the immune system more sensitive to the allergen for which it was made.

The National Institute of Allergy and Infectious Diseases provides an excellent PDF file on Understanding the Immune System.

Go to the nasal allergy HOME PAGE from the allergic reaction.

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