What Is IgM? - Deficiency, Antibodies & Blood Test

Instructor: Meghan Greenwood

Meghan has taught undergraduate and graduate level science courses and has a PhD in Immunology.

This lesson will describe the basics of antibodies, including more detail on one particular antibody class, IgM. It will also provide information regarding IgM deficiency as well as the laboratory blood test used to detect the antibodies.

What is an Antibody?

Think of the last time you got sick. Maybe the lymph nodes in your neck swelled up? That lump you felt underneath your chin is a way in which your immune system defends against infections.

The immune system is made up of many different cells, all with a common purpose of knowing when a bug, or pathogen, has entered your body and subsequently getting rid of it. One particular type of immune cell is called a B cell ('B' for bone marrow, where the cell develops).

B cells can be thought of as spaceships, traveling through the body looking for danger. When they encounter an intruder in your lymph node, they grow and fire blasts of antibodies, creating that lump you can feel in your neck. Antibodies are blood proteins made by B cells that act to neutralize the bug and alarm other cells to clear the infection.

Antibodies are unique for every intruder; they bind to a piece of the intruder called an antigen, which is created after the full pathogen is chewed up by various cells of the immune system. Each antibody is specific for each antigen. This specificity occurs through a whole other process, which we will not cover in this lesson.

In the image below, the antibody pictured can only bind to the antigen that fits, as colored in yellow. It's as if all the antibodies need to be a different key to fit into each pathogen lock. Once the antibody binds to an antigen, this sounds the alarm for other cells in the body to make more of these antibodies and clear the bug from circulation.

Antibody-Antigen specificity
antibody and antigen interaction

IgM: The Default Antibody

There are 5 main types of antibodies that can be produced by B cells: IgM, IgG, IgE, IgD and IgA. The Ig stands for immunoglobulin, which is the formal medical term for antibody and the letters represent different classes. Here we will only focus on IgM.

IgM is the default antibody made by B cells. It's considered the default because it is the first class produced in response to a new infection. All other classes are formed later, as the B cell becomes more mature and encounters more intruders.

Unlike the other types, IgM is not only blasted out of the B cell, but it also can stay attached to the surface of the B cell.

  • When IgM remains on the B cell surface, it is called the B cell receptor. This receptor binds to antigens and internalizes them, causing the B cell to grow, divide, interact with other immune cells, and eventually start secreting antibodies.
  • The IgM that is released from the B cell quickly clumps together with 4 other IgM molecules, forming a star pattern or pentamer, as pictured below. The glue in the center that holds the pentamer together is the antigen.

IgM pentamer- the antigen is in the middle in purple.
IgM pentamer

Detection and Deficiency

You have probably undergone a blood test before that measured your antibody titer. The titer means the amount of antibody you have for a specific pathogen. For example, if you've had chickenpox, your body produced antibodies against the virus. These antibodies will continuously be produced and remain in your body for many years. The titer will give the doctors an estimate of just how much antibody is in your system.

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