Inflammation and Hypersensitivity

We have seen that when a foreign virus or bacterium invades the body, there is a response in terms of cells and antibodies. To increase the effectiveness of this response the body assists as follows:

• blood vessels in the area are dilated, thus allowing more cells to get to the invader

• small holes appear in the walls of the blood vessels, allowing cells and plasma (the fluid part of blood) to leak out into the area

• plasma contains fibrin which can coagulate to form a sponge effect.

Externally these changes are seen as heat (increased blood flow) and swelling (plasma leaking into the tissue) and are called the changes of inflammation. Although they may be beneficial in counteracting disease, they cause discomfort to the animal. For example, inflammation in a leg due to entry of infection would lead to lameness.

Sometimes the inflammatory response can be so marked that it can be detrimental to the animal, or even be the cause of death. Probably the best example of this is in the lungs. Infection entering the lungs may produce such a marked inflammatory response, especially in terms of release of plasma, that the animal ‘drowns’ in the excess fluid and suffocates. In this instance we would need to give specific drugs (anti-inflammatory agents) to slow down the inflammatory response and to promote recovery.

On other occasions an animal over-reacts to the presence of a foreign invader, for example to a drug or vaccine injection. The immune system throughout the body may start to react and the animal will be seen shivering and shaking, perhaps frothing at the mouth, and eventually collapsing and possibly dying. This is known as an allergic or anaphylactic reaction and we say that the animal has a hypersensitivity to that particular invading antigen.