How do immune defenses work together?

A microbe crossing a protective barrier does not meet just one kind of defense. Immune cells can recognize warning signs, send chemical messages and recruit other cells. Some responses act broadly and quickly; others build a response to particular molecular features. The defenses overlap and communicate rather than waiting in a fixed queue.

Innate defenses respond to broad signs of microbes or tissue damage. Some cells engulf material, while chemical signals help draw other defenses to the area. Certain cells also display pieces of the material they encountered. Such recognizable molecular features are called antigens. Displaying them helps pass information from a local encounter to cells capable of a more specific response.

B cells and T cells are types of white blood cell in the adaptive immune system. Different cells recognize different antigen features. Suitable cells can become activated and multiply, increasing the number equipped to respond. Some B cells become factories for antibodies, proteins that bind particular features. Binding may block a microbe's attachment or mark it for other defenses. T cells help coordinate responses or act against infected cells, depending on their type.

After a response, some of these cell populations can persist as immune memory. A later encounter with a recognized feature can therefore begin with cells already prepared to respond more effectively. Memory does not guarantee that illness cannot occur, and its protection varies. The important connection is between recognition, communication and action: an antibody's binding is useful because it participates in a larger, cooperating defense system.