Biology · Introductory biology · Concept
Antibodies: how they recognize antigens
Antibodies are proteins, made by B cells, that bind specific antigens. Each antibody is a Y built from two identical heavy chains and two identical light chains joined by disulfide bonds. The tips of the arms are variable regions whose shape fits one epitope, a small part of the antigen; the stem, the Fc region, is constant within a class and links the antibody to the rest of the immune system. Binding neutralizes toxins and viruses, clumps pathogens together and marks them for destruction.
Antigens and epitopes
An antigen is any molecule an antibody can bind, often a protein or polysaccharide on a pathogen’s surface. An antibody does not bind the whole antigen: it binds an epitope, a small region of the antigen’s surface. One antigen can carry many different epitopes, each recognized by different antibodies.
The Y-shaped structure
Two heavy chains and two light chains, linked by disulfide bonds, form a Y. Each arm ends in the variable regions of one heavy and one light chain, which together form an antigen-binding site, so a Y has two identical sites. The stem, the Fc region, is made of the heavy chains’ constant regions.
| Part | What it is | Role |
|---|---|---|
| Heavy chains | Two identical long chains | Form the stem and the inner half of each arm |
| Light chains | Two identical short chains | Form the outer half of each arm |
| Variable regions | The tips of the heavy and light chains | Shape the antigen-binding sites |
| Antigen-binding sites | One at each arm tip | Each binds one epitope |
| Constant regions | The rest of each chain | Set the class and its job |
| Fc region | The stem | Binds immune cells and complement |
Variable and constant regions
Variable regions differ from one antibody to the next, which lets the body make antibodies against an enormous range of epitopes. Constant regions are the same within a class and decide what an antibody does once it binds: which cells it attaches to and where in the body it works.
Antibody classes
Humans make five classes of antibody, which differ in their constant regions and in how many Y units they join.
| Class | Form | Binding sites | Where and what |
|---|---|---|---|
| IgG | Monomer | 2 | Most abundant in blood; crosses the placenta |
| IgM | Pentamer | 10 | First made in a response; good at clumping |
| IgA | Dimer in secretions | 4 | Mucus, saliva, tears and milk |
| IgE | Monomer | 2 | Allergies and defense against parasites |
| IgD | Monomer | 2 | On B cells, as a receptor |
What binding does
Antibodies neutralize a toxin or virus by covering the parts it uses to attach to cells. Because each antibody has at least two binding sites, antibodies can link many antigens into clumps, called agglutination. Bound antibodies also mark a pathogen for phagocytes to engulf and set off the complement system.
Common mistakes
- Saying an antibody binds a whole pathogen or antigen: it binds one epitope, a small part of the surface.
- Thinking the two arms of one antibody bind different epitopes: both binding sites are identical and bind the same epitope.
- Mixing up antigen and antibody: the antigen is what gets recognized; the antibody is the protein that recognizes it.
- Placing the binding site on the stem: antigens bind at the arm tips, in the variable regions; the Fc stem is constant.
Key terms
- Antibody
- An immune-system protein that binds particular molecular features through variable regions. Binding specificity and the constant region contribute different functions.
- Antigen
- A molecule or molecular structure recognized by an immune receptor. The particular recognized region is an epitope, not necessarily the whole antigen.
- Epitope
- The specific part of an antigen contacted by an antibody or other immune receptor. One antigen can present several different epitopes.
- Antibody heavy and light chains
- A conventional antibody unit contains two heavy and two light polypeptide chains. Both contribute variable regions, while heavy-chain constant regions help determine antibody class.
- Antibody variable region
- The antibody regions contributing to antigen recognition. A Fab fragment includes an antigen-binding portion; a drawn fit does not measure actual binding strength.
- Antibody constant region
- Antibody chains contain constant regions as well as variable regions. The heavy-chain constant region contributes immune effector functions and helps define antibody class.
- B cell
- A lymphocyte that makes antibodies. Each B cell makes antibodies to one epitope; once activated it divides, and its plasma cells secrete large amounts of that antibody.
- Antibody neutralization
- Reduction of a target’s activity through antibody binding, such as blocking attachment. Binding alone does not demonstrate protective effectiveness in a biological system.
- Agglutination
- Clumping produced when binding molecules link separate particles or cells. Antibody-mediated agglutination requires an arrangement permitting such cross-linking.
Work through an example
An IgG molecule has two heavy and two light chains. How many antigen-binding sites does it have, and how many do pentameric IgM and secreted, dimeric IgA have? A lab cuts IgG with the enzyme papain into two Fab fragments, the arms, and one Fc fragment, the stem. How many binding sites does each Fab fragment have, and can Fab fragments clump antigens together?
Count antigen-binding sites →Sources and scope
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