Immune system proteins bind pathogens or signal other cells

The cells that defend mammalian bodies work together, interacting with one another and with the cells of invading pathogens. These cell–cell interactions are accomplished by a variety of key proteins, including receptors, other cell surface proteins, and signaling molecules. Four of the major players are listed here, and will be discussed in more detail later in the chapter.

  1. Antibodies are proteins that bind specifically to certain substances identified by the immune system as nonself. The specific molecules that bind to antibodies are called antigens. Antibodies are produced by B cells.

  2. Major histocompatibility complex (MHC) proteins are used to display antigens on the surfaces of self cells, so that the antigens can be detected by the T cells of the immune system. MHC proteins also function as important self-identifying labels.

  3. T cell receptors are integral membrane proteins on the surfaces of T cells. They recognize and bind to antigens presented by the MHC proteins on the surfaces of other cells.

  4. Cytokines are soluble signaling proteins released by many cell types. They bind to cell surface receptors and alter the behavior of their target cells.