Inflammation that has a slow onset and persists for weeks or more is referred as chronic inflammation.
The symptoms are not as severe as with acute inflammation, but the condition is insidious and persistent. Chronic inflammation may follow on acute inflammation or exist by itself.
An acute inflammation becomes chronic, if the immune system fails to clear the offending foreign agents from the body eg. Asbestosis.
Acute inflammation due to persistent infections such as tuberculosis, auto antigens responsible for certain autoimmune diseases such as rheumatoid arthritis also induce chronic inflammation in some people.
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Appearance of a mononuclear cell infiltrate composed of macrophages and lymphocytes, presence of granulation tissue are the characteristic features of chronic inflammation.
Macrophages are involved in microbial killing, in clearing up cellular and tissue debris, and they also seem to be very important in remodeling the tissues. The activities of Macrophages and Lymphocytes are interdependent.
Activation of one stimulates the actions of the other. In cases of chronic inflammation due to the presence of foreign agent macrophages can fuse together to form multinucleated/giant cells. For example Tuberculosis cause macrophage cells to unite in this manner.
Collagen production is a key feature of chronic inflammation. If too much collagen is formed, this can lead to the condition known as fibrosis.
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Connective tissue cells known as fibroblasts enter the area of tissue injury and produce collagen which is necessary to replace the tissue lost during long term inflammation.
The over production of collagen can lead to scarring that can cause permanent distortion of the tissue, interfering with its function. Chronic inflammation can be continually stimulated by substances with low antigenic properties or by autoimmunity.