The primary responses of naive CD4+ T cells are initiated in the peripheral lymphoid organs, to which protein antigens are transported after being collected from their portal of entry (Fig. 6-3). The common routes through which foreign antigens, such as microbes, enter a host are the skin and the epithelia of the gastrointestinal and respiratory systems. In addition, microbial antigens may be produced in any tissue that has been colonized or infected by a microbe. The skin, mucosal epithelia, and parenchymal organs contain numerous lymphatic capillaries that drain lymph from these sites and into the regional lymph nodes. Some antigens are transported in the lymph by APCs, primarily dendritic cells, that capture the antigen and enter lymphatic vessels, and other antigens may be in a free form. Thus, the lymph contains a sampling of all the soluble and cell-associated antigens present in tissues. The antigens become concentrated in lymph nodes, which are interposed along lymphatic vessels and act as filters that sample the lymph before it reaches the blood (see Chapter 2). Antigens that enter the blood stream may be similarly sampled by the spleen.
The cells that are designed to capture, transport, and present antigens to T cells are the dendritic cells. We next describe their major characteristics and their functions in initiating T cell responses.
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