Fever

Fever is the natural response to infection.
Pyrexia
Pyrexia refers to an increase in body temperature resulting in a fever. A person
with a fever is said to be febrile. When the body temperature is not raised the
person is described as being apyrexial (the prefix ‘a’ or ‘an’ always means without).
It is the hypothalamus which regulates body temperature by comparing the
temperature of the blood circulating through the structure with a set point,
usually between 36.5-37oC. When blood temperature drops, the hypothalamus
initiates mechanisms to increase body temperature; conversely when blood
temperature rises above the set point, heat loss mechanisms are initiated. In a
fever the set point is increased to a new higher setting.
Cause of fevers
Fevers are most commonly caused by infection but also occur after tissue damage
which may be caused by trauma, surgery or disease processes. For example, pyrexia
is not uncommon after a myocardial infarction when part of the myocardium
is damaged by an occlusion of the coronary arterial blood supply. Neoplasm is
another possible cause and fever may be an early sign of a developing cancer.
Infection is caused by the presence of microorganisms, such as bacteria or
viruses, in a tissue. As the presence of bacteria or viruses causes a febrile response
these infectious agents are often referred to as exogenous pyrogens. Exogenous
means the cause is from outside the body, the infection comes from the
environment. ‘Pyro’ literally means fire or heat, ‘gen’ refers to genesis which
means beginning, so a pyrogen is something which ‘begins heat’.
Leucocytes (white blood cells) are able to detect the presence of infectious
agents in a tissue. Upon detecting infection these white cells, especially the
macrophages, respond by producing cytokines. Cytokines are hormones
produced by individual cells which have the potential to affect other cells or
tissues in the body. Two of the cytokines produced by macrophages in response
to the presence of ..