The Role of Antigen Processing and Presentation

Cooperation in Immune Reactions to Antigens

Figure 2. Overview of the origins and events of adaptive immune responses.

The basis for most immune responses is the encounter between antigens and white blood cells. Microbes and other foreign substances enter most often through the respiratory or gastrointestinal mucosa and less frequently through other mucous membranes or the skin. Antigens introduced intravenously become localized in the liver, spleen, bone marrow, kidney, and lung. If introduced by some other route, antigens are carried in lymphatic fluid and concentrated by the lymph nodes. The lymph nodes and spleen are important in concentrating the antigens in areas where they will come in contact with antigen-presenting cells (APCs) and lymphocytes. APCs and lymphocytes can subsequently circulate into fluid compartments to seek out the antigens for which they are specific.

In most immune reactions, the antigen is in a “raw” state and must be further acted upon by antigen-presenting cells (APCs) before it is presented to T cells. Three different cells can serve as APCs: macrophages, dendritic cells ( dendritic Gr. dendron, branching like a tree), and B cells. Antigen-presenting cells modify the antigen so that it will be more immunogenic and recognizable. After processing is complete, the antigen is moved to the surface of the APC and bound to an MHC class II receptor to make it readily accessible to T cells during presentation (figure 3).

Before a T cell can respond to APC-bound antigens, certain conditions must be met. T-cell-dependent antigens, usually proteinbased, require recognition steps between the APC, antigen, and lymphocytes. The first cells on the scene to assist in activating B cells and other T cells are a special class of T helper cells (TH). The T cell receptor (TCR) of this class of T cell will bind simultaneously with the class II MHC receptor on the APC and with the antigen (figure 3). A second interaction involves the binding of the T-cell CD4 receptor to the MHC of the APC. Once this identification has occurred, a cytokine, interleukin-1 (IL-1), produced by the APC, activates this T helper cell. The TH cell, in turn, produces a different cytokine, interleukin-2 (IL-2), that stimulates a general increase in activity of committed B and T cells. The manner in which B and T cells subsequently become activated by the APC–T helper cell complex and their individual responses to antigen are addressed in the next two sections.

Дата добавления: 2015-06-04