Histamine and others
Histamine doesn't damage the pathogen as such but aids in the recruitment of other immune cells from the blood by dilating blood vessels. It also makes blood vessels more permeable to allow plasma proteins, such as antibodies, into the tissue to opsonise pathogens. Dilated blood vessels can be seen as redness at sites of inflammation whereas plasma proteins leaking in to the tissue causes swelling. Histamine also works on nerves to stimulate an itch in the skin. Have you ever wondered what itching is good for? Well, it may be a mechanism to physically remove parasites. Overall histamine is a good thing to have around as it induces swelling, redness and itching which all aid pathogen elimination. But hang on!, these same symptoms are seen in the allergic "wheal and flare" reaction! Have you made the connection yet?
The list of pre-stored mediators released during degranulation doesn't stop here! There are matrix metalloproteinases that breakdown the surrounding tissue to make room for other pathogen-killing cells, and lactoferrin, that bind iron molecules which are essential for replication of bacteria and thereby stop bacterial propagation, and the list goes on and on...The net effect of all these mediators is to make the environment which the pathogen sits in as inhospitable as possible and to quickly and efficiently eradicate the pathogen - too many cooks do not spoil the broth!! The problem with many of these mediators is that they can't discriminate between pathogenic and host cells and therefore cause a lot of tissue damage! Both MBP and matrix metalloproteinases are elevated in asthma and are thought to breakdown tissue leading to airway remodelling.