What is an Antibody? - Definition, Structure & Function

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

In this lesson, we will explore your immune system and how it deals with invaders, such as microbes and bacteria. We will define antibodies and discuss their structure and how this relates to their function and then you can take a short quiz!

Definition of Antibody

You are sick again - fever, chills, vomiting. You might not think it's doing anything, but your immune system is working hard to destroy whatever unwanted microbe is inhabiting your body. Let's explore your immune system, focusing on the antibody, or the part of your immune system that finds and puts a hit out on microbes. Think Al Capone of the immune system.

White Blood Cells

Before we get to antibodies, let's explore the cells that make the antibodies. Your immune system is made up of organs, cells and tissues including a type of white blood cells called lymphocytes. There are two types of lymphocytes: B-lymphocytes and T-lymphocytes.

  • B-lymphocytes are the cells that are alerted when 'bad guys' enter your body (primarily the microbes that make you sick). These 'bad guys' have proteins on their surface called antigens. Think of B-lymphocytes and the antibodies they produce as the mob boss.
  • T-lymphocytes come into play when the B-lymphocytes find the bad guys, and they tell T-lymphocytes, which take care of business. Think of T-lymphocytes as the mob boss's crew.

A scanning electron microscope image of a T-lymphocyte
T Cell

It may be tricky to keep the B and T separate so I always think of the B-lymphocytes as the Boss that tell the T-lymphocytes what to do. In reality, they get their name because B-lymphocytes can be found in bone marrow and T-lymphocytes can be found in the thymus.

Invaders: Immune System Response

In order to go through the immune process, let's have a harmful bacteria enter your body. You are going to have enough vocabulary, so let's call the bacteria something easy like Bobby Bacteria. Now Bobby will have lots of bacteria friends but let's just focus on Bobby.

Bobby enters your body through a cut on your hand. Pretty soon other cells in the body recognize proteins on Bobby as antigens, and they alert the B-lymphocytes, which start producing antibodies.

The Antibody Structure

Don't worry, we'll get back to Bobby shortly but we need to explore the antibody in more detail.

The antibody is in the shape of a Y, and it is made up of proteins. The tips of the Y can vary greatly from antibody to antibody. The antigen, or the proteins on microbes like Bobby, also vary greatly from microbe to microbe. In order to flag microbes, the antibodies must match up to the antigen, kind of like a lock and a key.

It's the job of the B-lymphocytes to make the right antibody that will fit the antigen. This is why the tips of the Y vary greatly - each type of antibody is specific to each type of antigen.

In order to remember the difference between antibody and antigen, think of this antibodies are made by the body.

The structure of an antibody. Notice the Y shape. The tips (colored darker green and blue) vary greatly from antibody to antibody.

Antibodies are Produced

So, back to Bobby. The B-lymphocytes now work feverishly to make antibodies in order to make Bobby a target. Remember, they need to make an antibody that will fit the antigens on Bobby so it may take a little time but, eventually, they have success. Antibodies are sent out and they attach onto Bobby's antigens and, although the antibodies can't kill Bobby, it's only a matter of time.

The white blob is a B-lymphocyte and the yellow Y shapes are antibodies tagging the antigens on the green bacteria.

Pretty soon the crew, T-lymphocytes, see that Bobby is marked and they kill Bobby. So long Bobby Bacteria.

Notice the antibody and antigen fit together like a lock and key. Each unique antigen must have a unique antibody,

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