antibody
Meaning of antibody: Five classes exist, and which one is present says a good deal about where an immune response is and how old it is.
Definition of antibody
An antibody is a Y shaped protein the immune system builds to lock onto one specific target, called an antigen. Plasma cells, which develop from B lymphocytes, manufacture them. Each antibody fits its antigen the way a key fits one lock, and that precise fit is what lets the body tell an intruder apart from its own tissue.
What happens once an antibody binds?
Binding is the start, not the end. The two arms of the Y grip the antigen, and the stem then decides what follows. Neutralization simply blocks the target, coating a virus so it cannot dock onto a cell or tying up a bacterial toxin so it cannot act. Opsonization coats the target in a layer that phagocytes recognize and swallow, turning a scattered threat into food. Activation of complement sets off a cascade of blood proteins that punches holes in bacterial membranes and calls in more cells. Antibodies also flag a coated cell for destruction by natural killer cells. The immune system therefore gets several different weapons from one recognition event.
What are the five classes?
| Class | Where it works | What it signals |
|---|---|---|
| IgM | Bloodstream, in large star shaped clusters | An early or recent response |
| IgG | Blood and tissue fluid; crosses the placenta | An established or past response, and lasting protection |
| IgA | Saliva, tears, gut and airway lining, breast milk | Defense at surfaces exposed to the outside |
| IgE | Bound to mast cells in tissue | Allergy and parasitic infection |
| IgD | Mostly on the surface of B cells | A role in switching B cells on |
That pattern is why testing often reports two results for the same organism. IgM without IgG suggests a first encounter that is still under way; IgG alone suggests exposure some time ago or vaccination. It also explains why a newborn carries a mother's IgG for months while making very little of its own.
Where do laboratory made antibodies come from?
Antibodies can be produced outside the body and given as medicines. A monoclonal antibody is grown from a single clone of cells, so every molecule is identical and aimed at exactly one target. These are used to block a receptor, mark a cancer cell for the immune system, or carry a drug or radioactive atom to a chosen site, and they carry the same specificity that makes natural antibodies useful.
Also known as
Used in a sentence
Blood testing detected antibodies consistent with prior exposure to the virus.
Good to know
An antibody recognizes a single antigen, which is what separates the adaptive immune response from broader defenses such as inflammation that act without targeting a particular organism.