TH2 cells secrete IL-4, IL-5, and IL-13, which recruit and activate eosinophils and are responsible for the alternative pathway of macrophage activation.
TH17 cells secrete IL-17 and other cytokines, which induce the secretion of chemokines responsible for recruiting neutrophils into the reaction.
Both TH1 and TH17 cells are involved in defense against many types of bacteria and viruses and in autoimmune diseases. TH2 cells are important in defense against helminthic parasites and in allergic inflammation. These T cell http://ebooksmedicine.net (e.g., IL-12,
IL-6,  IL-23)
Activated ActivatedT lymphocyte macrophage(TH1, TH17)
Presents antigen to T cells
TNF,IL-17, IL-1,TNF chemokines subsets and their functions are described in more detail in Chapter 5.
Lymphocytes and macrophages interact in a bidirectional way, and these interactions play an important role in propagating chronic inflammation (
Fig. 3.20 ). Macrophages display antigens to T cells, express membrane  molecules (called costimulators) that activate T cells,  and produce cytokines (IL-12 and others) that also  stimulate T cell responses (Chapter 5). Activated T lymphocytes, in turn, produce cytokines, described earlier, which recruit and activate macrophages, promoting more antigen presentation and cytokine secretion. The result is a cycle of cellular reactions that fuel and sustain chronic inflammation.
Activated B lymphocytes and antibody-producing plasma cells are often present at sites of chronic inflammation. The antibodies may be specific for persistent foreign or self antigens in the inflammatory site or against altered tissue components. However, the specificity and even the importance of antibodies in most chronic inflammatory disorders are unclear.
In some chronic inflammatory reactions, the accumulated lymphocytes, antigen-presenting cells, and plasma cells cluster together to form lymphoid structures resembling the follicles found in lymph nodes. These are called tertiary lymphoid organs; this type of lymphoid organogenesis is often seen in the synovium of patients with longstanding rheumatoid arthritis, in the thyroid in Hashimoto thyroiditis, and in the gastric mucosa in the setting of Helicobacter pylori infection. It has been postulated that the local formation of lymphoid organs may perpetuate the immune reaction, but the significance of these structures is not established.
Other cell types may be prominent in chronic inflammation induced by particular stimuli.
• Eosinophils are abundant in immune reactions mediated by IgE and in parasitic infections (
Fig. 3.21 ). Their recruitment is driven by adhesion molecules similar to those used by neutrophils, and by specific chemokines (e.g., eotaxin) derived from leukocytes and epithelial cells. Eosinophils have granules that contain major basic protein, a highly cationic protein that is toxic to parasites but also injures host epithelial cells. This is why eosinophils are of benefit in controlling parasitic infections, yet also contribute to tissue damage in immune reactions such as allergies (Chapter 5).