First of all, the little blood-vessels or capillaries supplying the injured part dilate, and thus more blood is brought into the area. The next happening can be observed through a microscope and is very dramatic to watch. To understand it we must know that blood is composed of a fluid substance in which float two types of cells, the red and the white. The white cells, or leucocytes, are the fighting forces of the blood, and they are now brought into action. As we watch, the blood circulating in the dilated vessels begins to flow more slowly, and the leucocytes line up in ranks along the vessel walls. They next thrust themselves through these walls by active movements and descend on the invading germs. The battle now begins: the white warrior cells have the power to kill the germs by seizing and engulfing them. The germs however, have also a weapon of defence – they manufacture poisonous substances or “toxins” which kill the body tissues and cause heavy casualties among the soldier cells. In order to dilute these toxins and render them less harmful, a fluid called lymph, which ordinarily circulates in little accessory channels near the blood-vessels, is poured on the scene of action; lymph also contains “anti-toxins” to neutralize the germ-poison.
At this stage the defence may overcome the germs present; and the resulting debris, which consists of the dead germs, the tissue cells which have been destroyed by them and those of the white cells that have died during the struggle, will be removed from the scene. This is the work of the small lymph channels, which carry the debris to the nearest glands, where it is destroyed. Repair of the broken blood vessels and damaged tissues now starts by a multiplication of the healthy cells which surround the area, until the part once more assumes its normal appearance.
If the germs are victorious, the struggle will continue longer, and matter or pus will be formed. Pus is, as we know, yellow and thick in consistency, and when it is present we talk of the condition being septic. There is good reason for this, as pus is really a concentrated mass of germs and the cells which have been killed by them, most of the latter being of the white “fighting” type. When pus forms, the body cannot dispose of it as it does in the case of the simple inflammatory debris, but it succeeds, as a rule, in localizing it, and when this happens, under the skin a boil or abscess results. Localized pus always works towards the surface of the body so that it can escape by discharging; this in the case of a boil, may be a long, painful process, and recovery can be hastened and pain relieved by lancing.