The principal chemical mediator of immediate phase, of acute inflammation is:

1. Serotonin

2. Histamine

3. Kinin-Kallikrein

4. Complement system

The principal chemical mediator of the immediate phase of acute inflammation
is Histamine. Here's a detailed explanation of the options given:

1. Serotonin: While serotonin is a vasoactive substance that can cause blood
vessels to constrict or dilate, it is not the primary mediator of the immediate
phase of acute inflammation. It is mainly associated with the regulation of
mood, appetite, and sleep. In the context of inflammation, it plays a minor role
compared to histamine.

2. Histamine: Histamine is indeed the correct answer. It is a potent chemical
mediator released from mast cells and basophils in response to injury or
antigenic stimulation. Upon release, histamine acts on blood vessels to cause
vasodilation, increased permeability, and increased blood flow to the injured
area, which are hallmark features of the immediate phase of acute inflammation.
This results in the cardinal signs of inflammation: redness (rubor), heat
(calor), swelling (tumor), and pain (dolor).

3. Kinin-Kallikrein system: The kinin-kallikrein system is another important
mediator of inflammation, but it is more involved in the later phases. When
activated, it results in the formation of kinins, such as bradykinin, which
contribute to increased vascular permeability and pain. However, it is not the
first line mediator in the immediate phase.

4. Complement system: The complement system is a group of proteins in the blood
that work with antibodies to destroy pathogens and trigger inflammation. It is a
key component of the innate immune response, but its activation and role are
more pronounced in the later stages of inflammation rather than the immediate
phase. The complement system is involved in the opsonization of pathogens,
recruitment of phagocytes, and the formation of the membrane attack complex,
which can lyse certain bacteria and cells.

The immediate phase of acute inflammation is characterized by the rapid response
to tissue injury, which includes vasoactive changes and increased vascular
permeability to allow fluid, cells, and proteins to move into the interstitial
space. Histamine is quickly released from mast cells and basophils and acts on
H1 receptors of blood vessels to induce vasodilation and increased permeability.
This leads to the early symptoms of inflammation, such as swelling, redness,
heat, and pain, and is crucial for the initiation of the inflammatory response
to protect the body from harm.