Antibodies are called immunoglobulins because they are, broadly speaking, the same type of protein, and immunoglobulin is a broader term that encompasses both the secreted form (often specifically called antibody) and the membrane-bound form.
Immunoglobulins (Ig) represent a family of proteins produced by the immune system, specifically by plasma cells derived from B lymphocytes. These proteins play a crucial role in recognizing and neutralizing foreign substances like bacteria and viruses.
The Relationship Between Antibody and Immunoglobulin
While the terms are frequently used interchangeably, there is a subtle distinction, as highlighted by the definition:
- Antibody (Ab): Can refer more narrowly to the free, secreted form of these proteins that circulate in the blood and other bodily fluids.
- Immunoglobulin (Ig): Can refer to both the free, secreted form (the antibody) and the membrane-bound form which functions as the B cell receptor (BCR) on the surface of B lymphocytes.
Since both the secreted antibody and the membrane-bound B cell receptor are fundamentally the same protein structure with the same antigen-binding specificity (once a B cell is activated), they are considered "broadly speaking, the same protein." This core similarity is why the terms immunoglobulin and antibody are often treated as synonymous in many contexts.
Here's a simple way to look at it:
| Term | Refers To | Location |
|---|---|---|
| Immunoglobulin | Both secreted and membrane-bound forms | Blood, fluids, and surface of B cells |
| Antibody | Primarily the secreted form (more narrowly) | Blood, fluids (can also loosely mean the molecule) |
In essence, all antibodies are immunoglobulins, but not all immunoglobulins (specifically, the membrane-bound form) are typically referred to only as "antibodies" when making a strict distinction. However, due to their fundamental identity, using "immunoglobulin" or "antibody" to refer to the secreted protein is perfectly acceptable and common practice.
This dual naming reflects their critical role in immunity ("immuno") and their protein nature ("globulin"), which refers to a large class of globular proteins. Thus, immunoglobulin literally means a globular protein involved in immunity.
