II. Cellular Events

The cellular phase of inflammation consists of 2 processes:

1. exudation of leucocytes; and

2. phagocytosis.

EXUDATION OF LEUCOCYTES

The escape of leucocytes from the blood vessels is the most important feature of inflammatory re­sponse. In acute inflammation, polymorphonuclear neutrophils (PMNs) comprise the first line of body defense, followed later by monocytes and macrophages.

PHAGOCYTOSIS

Phagocytosis is defined as the process of engulfment of solid particulate material by the cells (cell-eating). The cells performing this function are called phagocytes. There are 2 main types of phagocytic cells:

(i) Polymorphonuclear neutrophils (PMNs) which appear early in acute inflammatory response, also called as microphages.

(ii) Circulating monocytes and fixed tissue mononuclear phagocytes called as macrophages.

The process of phagocytosis is similar for both polymorphs and macrophages and involves the following 3 steps:

1. Attachment stage (Opsonisation)

2. Engulfment stage

3. Killing or degradation stage.

CHEMICAL MEDIATORS OF ACUTE INFLAMATION

Also called as permeability factors or endogenous mediators of increased vascular permeability, these are a large and increasing number of endogenous compounds which can enhance vascular perme­ability. However, currently many chemical me­diators have been identified which partake in other processes of acute inflammation like vasodilatation, chemotaxis, fever, pain and cause tissue damage.

The substances acting as chemical media­tors of inflammation may be released from the cells, the plasma, or damaged tissue itself. They are broadly classified into 2 groups:

I. Mediators released by cells; and

II. Mediators originating from plasma.

I. CELL-DERIVED MEDIATORS

1. Vasoactive amines (Histamine, 5-hydroxytryptamine)

2. Arachidonic acid metabolites

(i) Metabolites via cyclo-oxygenase pathway (prostaglandins, thromboxane A2, prostacyclin) (ii) Metabolites via lipo-oxygenase pathway (5-HETE, leukotrienes)

3. Lysosomal components

4. Platelet activating factor

5. Cytokines (interleukin 1, tumor necrosis factor)

II. PLASMA-DERIVED MEDIATORS

(PLASMA PROTEASES): These are products of:

1. The kinin system

2. The clotting system

3. The fibrinolytic system

4. The complement system

THE INFLAMMATORY CELLS

The cells participating in acute and chronic in­flammation are circulating leucocytes, plasma cells and tissue macrophages.

FACTORS DETERMINING VARIATION IN INFLAMMATORY RESPONSE

Although acute inflammation is typically characterized by vascular and cellular events with emigra­tion of neutrophilic leucocytes, not all examples of acute inflammation show infiltration by neutrophils, and vice versa, some chronic inflammatory condi­tions are characterized by neutrophilic infiltration. For example, typhoid fever is an example of acute inflammatory process but the cellular response in it is lymphocytic; osteomyelitis is an example of chronic inflammation but the cellular response in this condition is mainly neutrophilic.

The morphologic variation in inflammation depends upon a number of factors and processes. These are discussed below: