Each antigen has distinctive markers, characteristic shapes that protrude from its surface. It is by these markers, called epitopes, that antigens announce their foreignness and identify themselves to the immune system. Most antigens carry several different kinds of epitopes on their surface. Some epitopes are more easily recognized than others, and so are more effective at stimulating an immune response.

Both B cells and T cells carry customized receptor molecules that allow them to recognize and respond to their specific targets. This recognition is enabled by:

• Antibodies. Portions of antibodies serve as the receptors on B cells. When an antibody recognizes its antigen, it binds to a specific epitope, like interlocking pieces of a jigsaw puzzle, and may cover a portion of the antigen. The reaction of the antibody may then lead to one of the following events:
  • Other immune cells are summoned to the site to attack the bound antigen.
  • The cell bearing the antigen is destroyed in a process that is mediated by complement (described under Destroying Antigens).
  • The cell bearing an antigen and bound to an antibody may be taken up and destroyed by the immune system in a process known as opsonization.
• Antigen presenting cells (APCs). The T cell receptor does not recognize some antigens until that they are processed and presented by an antigen-presenting cell (APC). An APC is a specialized type of white blood cell that engulfs antigens and breaks them down into smaller fragments. These fragments are transported to the surface of the APC, where they are linked to special marker proteins known collectively as the major histocompatibility complex (MHC) and "presented" to the T cell. When the antigen is linked with MHC, the T cell receptor can now recognize the antigen and bind to it. Cytotoxic T (CD8) cells recognize antigens in the context of MHC class I molecules. Helper T (CD4) cells recognize antigens in the context of MHC class II molecules.

  The most powerful and effective APC is the dendritic cell. It gets its name from the Greek word dendron ("tree") because it resembles a tree, with roots and branches spreading out from the main body of the cell.