antigen-presenting cell
Meaning of antigen-presenting cell: Cells that decide whether an immune response begins, by carrying a fragment of an intruder to a lymph node for T cells to inspect.
Definition of antigen-presenting cell
An antigen-presenting cell is an immune cell that takes up material from outside itself, chops it into fragments, and displays those fragments on its surface where T cells can inspect them. The display sits in a groove on a surface protein built for the purpose. Dendritic cells, macrophages and B cells carry out this task as a main part of their work.
How does presentation work?
A T cell cannot see a whole bacterium or a virus particle. It reads only short fragments held out to it in the groove of a display protein, so something has to do the collecting and the chopping first. An antigen-presenting cell takes in material, breaks it down inside itself, loads the pieces into that groove and carries the result to the nearest lymph node, where T cells are gathered and circulating. A T cell whose receptor matches the fragment stops, engages and begins to divide, and the response to that antigen starts from there.
Which cells do it?
| Cell | How it collects material | What it does best |
|---|---|---|
| Dendritic cell | Samples surrounding fluid constantly | Starting a response from nothing |
| Macrophage | Engulfs microbes and dead cells | Sustaining a response within tissue |
| B cell | Captures with its own surface receptor | Recruiting help to make antibody |
| Any nucleated cell | Displays its own internal proteins | Revealing infection to killer T cells |
The last row explains a common muddle. Every nucleated cell in the body puts samples of its own contents on show, which is how a virus hiding inside one is found, but the phrase antigen-presenting cell is reserved for the cells in the rows above it. It is also not a synonym for phagocyte: a B cell presents antigen without engulfing anything, capturing it instead with the receptor on its surface.
Why does a second signal matter?
Because presentation on its own does the opposite of what might be expected. A T cell that meets its matching fragment without a second, confirming signal from the same cell is not activated but silenced, and may be destroyed. That arrangement is deliberate. Cells presenting fragments of ordinary body tissue, with nothing wrong nearby, deliver no second signal, so the immune system is continually taught to leave its own tissue alone. Infection or damage supplies the confirming signal, and only then does a response follow.
Also known as
Used in a sentence
The vaccine was designed to deliver the antigen directly to antigen-presenting cells.
Good to know
Presentation alone does not switch a T cell on, since without a second confirming signal from the same cell the T cell is silenced instead, which is one of the mechanisms that prevents attacks on the body's own tissue.