Most people are either naturally immune to the spores of Aspergillus fumigatus, or have a sufficiently healthy immune system to fight the infection. However, if you have an allergic reaction (see ABPA) to the fungal spores and/or have lung problems or a weakened immune system then you are particularly susceptible.
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Aspergillus species produce microscopically small spores which are extremely light and float in the air around us. This is how they spread. Normally when aspergillus spores are inhaled by people, their immune system is activated, the spores are recognised as foreign and they are destroyed - no infection results.
Occasionally in an individual with a weakened immune system the spores are not "seen" and they can grow inside a lung or wound. When this happens the patient has an illness called aspergillosis - there are several different types of aspergillosis more details.
A weakened immune system means that some immune responses which are normally switched on when a foreign microorganism or virus enters the body do not function properly - this may be due to chemotherapy, or to medicines taken after an organ or bone marrow transplant or because you have an inherited disorder affecting the immune system such as cystic fibrosis and CGD.
The white blood cells are cells which are able to recognise a foreign component in the tissues of the body and destroy it. An antibody is a special molecule which the body produces to help activate some of the specific cells present in the immune system - in order to recognise a foreign microbe such as aspergillus. They are called 4 types IgG, IgA, IgM and IgE. Antibodies against aspergillus proteins can be measured in a patient's blood and this is one test which indicates if the patient may have an aspergillus infection - this is sometimes called the aspergillus precipitins test. Another assay which measures whether a patient has had exposure to aspergillus proteins is called the galactomannan assay - where antibodies specific to an aspergillus cell wall molecule are tested in a blood sample.
Another measure that the immune system has been activated and a possible allergic-type of reaction has occurred, is to measure a patient's IgE levels - a significantly elevated level suggest immune activation- then the presence of IgE antibodies specifically to aspergillus species may be tested. This test will assist in the possible diagnosis of aspergillosis.