Cell Communication

In order for the various cells of the immune system to respond to trauma or invasion in a cooperative fashion, cells must communicate both with their immediate environment and with each other. Cells receive signals from their external environment by producing surface receptors that are able to bind specific chemical messengers; these surface receptors can be considered the "eyes" and "ears" of a cell. The "voices" of a cell are the cytokines, or chemical messengers, that a cell can make. In addition, some cells can make adhesion molecules; these function as "hands," enabling one cell to directly contact another.

Surface Receptors

Surface receptors are integral membrane proteins to which certain signal molecules bind. They generally span the cell membrane, connecting the outside of the cell with the inside, enabling the inner workings of the cell to sense and respond to signals outside of the cell. Each surface receptor is specific with respect to the compound or compounds it will bind; a molecule that can bind to a given receptor is called a ligand for that receptor. When a ligand binds to its surface receptor, the internal portion of the receptor becomes modified in some manner, effectively communicating to the cell that the ligand is present. This then elicits some type of response, such as chemotaxis. ■ chemotaxis, p. 65

Cells can alter the types of surface molecules they make, enabling them to respond only to signals that are relevant when the cell is in a certain location or developmental stage. For example, a dendritic cell in the tissues would respond differently to certain stimuli than one that has migrated to a secondary lymphoid organ.

Cytokines

Cytokines are low molecular weight proteins made by certain cells as a mechanism to communicate with other cells. Cytokines produced by lymphocytes are also called lymphokines. Cytokines bind to certain surface receptors, cytokine receptors, found on the cells they regulate, inducing a change such as growth, differentiation, movement, or cell death. Although cytokines are short-lived, they are very powerful, acting at extremely low concentrations. They can act locally, regionally, or systemically. Often, they act together or in sequence, in a complex fashion. The source and effects of representative cytokines are summarized in table 15.3. General characteristics of the various groups are briefly described here:

■ Chemokines are cytokines important in chemotaxis of immune cells; more than 50 different varieties have been identified by their structure. Certain types of cells of the

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Chapter 15 The Innate Immune Response

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