Question

Why do lymphocytes activated in mucosal tissues home back there?

Why can superantigens activate a large number of T cells?

Answer 1

1. Lymphocytes activated in mucosal tissues enter the lymphatics draining the intestines and reach in the thoracic duct, from where they circulate in the blood throughout the entire body . When they reach small blood vessels lining the gut wall and other sites of MALT; these vessels express the mucosal adressin MAdCAM-1. Mucosal addressin which is selectively expressed on mucosal tissues direct the activated lymphocytes to re-enter mucosal tissue. In this way, an immune response that started in the mucosal tissue of of one site is disseminated throughout the mucosa of the body and confined to mucosal tissues.

2. Superantigens bind with Tcells via  T-cell receptor (TCR) and major histocompatibility complex (MHC) class II molecules.

They selectively stimulate T cells expressing certain TCR Vβ chains

Superantigen can activate a large proportion of the T-cell population because

• the human T-cell repertoire comprises only about 50 types of Vβ elements
• some superantigens are able to bind multiple types of Vβ regions