The Histology Guide

Inflammatory Response

An inflammatory reaction occurs in response to a local tissue injury, to reverse the effects and repair the damage. Typically, the damaged area becomes red and hot, and the underlying connective tissue swells.

This reaction happens in connective tissue, and involves neutrophils, monocytes, mast cells, and endothelial cells of venules.

First: blood flow is increased, and venules and capillaries dilate. Plasma leaks from the affected venules, resulting in swelling (oedema). Histamine release, causes the endothelial cells of the venules to separate slightly, which allows proteins, such as fibrinogen, immunoglobules, etc to enter into the connective tissue.

Neutrophils adhere to the endothelial cells, and migrate into the connective tissue, by squeezing between them, and then chemotax (follow a chemical trail) towards bacteria, which they phagocytose and destroy.

Monocytes also enter the tissue in a similar way, and transform into macrophages, that phagocytose some types of bacteria, and dead cells. Macrophages also release substances called cytokines, the attract other cells, mediate inflamation, and promote tissue repair.

To repair the damage, fibroblasts actively secrete pro-collagen, blood supply becomes re-established (angiogenesis), and some fibroblasts transform into myofibroblasts, that help to contract the connective tissue, and reduce the size of the scar.