Meghan has taught undergraduate and graduate level science courses and has a PhD in Immunology.
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.
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.
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.
As mentioned above, different pathogens are associated with different antibody classes. Since IgM is the first antibody produced, it may also help inform the doctor of a new infection. For example, if you are experiencing symptoms and need a diagnosis, the IgM test would be ordered.
In addition to specific antibody titers, a doctor may also look at your overall IgM levels. A lack of IgM below a certain threshold may signal to the doctor that you have an immunodeficiency. IgM deficiency can occur because of your genetic background or as a result of other diseases, like cancer or an autoimmune disease. Although this condition is rare, it could increase your susceptibility to infections or even sepsis, a dangerous blood infection. The cause of IgM deficiency is relatively unknown, but thankfully, less than 1% of the population is known to have it.
Antibodies are molecules made by B cells to attack pathogens (intruders). They bind to pieces of pathogens, called antigens, and act to neutralize the bug and alarm other cells to clear the infection. Antibodies come in five classes, with IgM being the first antibody produced by the B cells. IgM binds to antigen in a groups of five, called a pentamer. The doctor will check your IgM levels for certain diseases and to diagnose specific symptoms. IgM deficiency, or a lack of normal IgM levels, is a rare disorder that can cause an increase in risk of infection.
Medical Disclaimer: The information on this site is for your information only and is not a substitute for professional medical advice.
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