What is IgD? - Definition & Function

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  • 0:02 Intro: Antibody Function
  • 1:46 Definition of IgD
  • 3:28 Function of IgD
  • 6:17 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

This lesson will provide a quick review of antibodies. Then you will discover what IgD means, what it is, and where it comes from. You will also learn what is known about its role in the body.

Intro: Antibody Function

Fevers, coughing, sneezing, and vomiting: Being sick is the pits. Why do we get sick anyways? Well, getting sick is our body's way of trying to combat enemy invaders trying to take over our cells. Invaders come in many shapes, sizes, and forms: bacteria, viruses, and even parasites. Our body responds to invaders with a huge defense army known as the immune system. The immune system is composed of many different types of cells that react differently to invaders, each trying in its own way to kill off the invading entity.

For example, B-cells, produced in the bone marrow, travel around the body looking for invaders. They may also learn about them from other members of the immune system. Their special tactical weapons include antibodies (Ab), also called immunoglobulins (Ig). Immunoglobulins are proteins, most often Y-shaped, which enter into hand-to-hand combat with the invader and stop it from infecting cells. Think of the Y as arms and hands grabbing the enemy.

We call the part of the invader the Ig grapples with the antigen (Ag). Each B-cell produces its own unique Ab that is specific for an Ag. When a B-cell learns that an enemy Ag matching its Ab has been found, it goes into attack mode and secretes, or sends out, Ab into the body.

Definition of IgD

Before B-cells enter attack mode, Ab attaches to the cellular membrane by the stem of the Y. There are five different types of stems, which make up the five different classes of Ab. The classes are called by their Ig names: IgA, IgD, IgE, IgG, and IgM. IgD stands for immunoglobulin D; right now, you probably have a little bit of IgD in the tissues of your chest or stomach.

Scientists who discovered IgD named it 'D' because it was distinct from the other three Igs known at that time. It turns out that D was a good name for this antibody because IgD is also very diverse. IgD is a very mysterious Ab. At first, scientists thought it had a Y-shaped structure like all other Ab, but some experiments indicate that IgD can also adapt a T-shaped structure.

When B-cells form, they express IgM on their cell membrane. When B-cells mature, they not only have IgM, but also exhibit IgD on their surfaces; this is called co-expression. Only after IgD becomes visible do the B-cells really go after invaders; both IgD and IgM on the B-cell can attack the same Ag. In healthy people, very little IgD is secreted.

Function of IgD

When mature B-cells go into attack mode, they choose the class of Ig they want to use to attack a specific Ag. This means that, when they get the call to action, B-cells stop co-expressing IgM and IgD and start making, expressing, and secreting only one kind of Ig, like IgG.

IgD can bind to many different enemies, including rubella, diphtheria, measles, and streptococcus (strep) bacteria. Interestingly, the places in the body these enemies can attack are also where more IgD is often secreted - places like the tonsils, saliva, nose mucus, and tears.

Only rarely do B-cells choose to make, express, or secrete only IgD when in battle mode. Why is this? That's a good question because the exact functions of IgD are still being uncovered. Much of what is known about the function of IgD comes from studying different disease states.

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