Asthma is often caused by allergens such as pollen, dust or animal hair. When an asthma sufferer is exposed to an allergen they are sensitive to, the early or “immediate” asthmatic response occurs. Approximately 3-8 hours after exposure to the allergen, a late phase response can occur, which can cause coughing and wheezing. It is during the late phase of asthma that eosinophils cause chronic inflammation of lung tissue.
During the late phase, eosinophils – a type of white blood cell – are recruited to the lungs by platelets. This occurs when a platelet receptor called P-selectin attaches to eosinophils in blood vessels surrounding the lungs, and activate α4β1 integrins on the eosinophil surface. This integrin activation allows the eosinophil to tightly adhere to the blood vessel wall, and transmigrate into the lung tissue (Johansson et al, 2012).
Once inside the lungs, the eosinophils release granules containing damaging chemicals such as major basic protein (MBP) and eosinophil cationic protein (ECP). These damage cells within the lung, causing long-lasting inflammation (Chihara et al, 1997). Part of the reason that the late phase response is long lasting is that platelets and eosinophils produce a chemokine called RANTES. This is a potent chemoattractant, and attracts more eosinophils to the lungs and induces them to degranulate once inside the lung (Filipovic and Cekic, 2001). This continues the cycle of cell damage and inflammation within the asthmatic lung.
Chihara et al (1997) “Elevation of the plasma level of RANTES during asthma attacks” Journal of Allergy and Clinical Immunology. Volume 100 S52-55
Filipovic and Cekic (2001) “The Role of Eosinophils in Asthma” Medicine and Biology Volume 8, No. 1 p. 6 – 10
Johansson et al (2012) “Platelet activation, P-selectin and Eosinophil β1-integrin Activation in Asthma” American Journal of Respiratory Cell and Molecular Biology. Volume 185 No. 5 p. 498–507.