ABSTRACT
The adaptive immune response occurs mainly in the secondary lymphoid tissue-the lymph nodes, the gut-associated lymphoid tissue, and the spleen (Fig. 1.1). Pathogens and their secreted antigens are trapped in these tissues, and presented to the naive lymphocytes that constantly pass through. Microorganisms that enter the body through the skin or the lungs drain to regional lymph nodes where they stimulate an immune response. Microorganisms and food antigens that enter the gastrointestinal tract are collected in the gut-associated lymphoid tissue. Microbes that enter the bloodstream stimulate an immune response in the spleen.
