FIGURE 23-3 Initiation of the adaptive immune response in lymph nodes. Recognition of antigen by B and T cells (lymphocytes) located in lymph nodes initiates an adaptive immune response. Lymphocytes leave the circulation and take up residence in lymph nodes (step 1). Lymph carries antigen in two forms, soluble antigen and antigen-laden dendritic cells; both are delivered to lymph nodes via afferent lymphatic vessels (steps 2 and 3). Soluble antigen is recognized by B cells (step 4), and antigen-laden dendritic cells present antigen to T cells (step 5). Productive interactions between T and B cells (step 6) allow B cells to move into follicles and differentiate into plasma cells, which produce large amounts of secreted immunoglobulins (antibodies). Efferent lymphatic vessels return lymph from the lymph node to the circulation.