Antibodies: structure and function

What are antibodies?

Antibodies are proteins your immune system makes to recognise and help destroy invaders such as bacteria and viruses. Every pathogen carries antigens: molecules on its surface, usually proteins, that the body recognises as foreign. Each type of antibody fits one particular antigen, like a key in a lock. Antibodies are also called immunoglobulins.

They are made by B lymphocytes, a type of white blood cell. When a B lymphocyte meets the antigen its antibody fits, it divides to make many copies of itself, and these become plasma cells that release huge numbers of identical antibodies into the blood.

Antibody structure

Because an antibody is made of four polypeptide chains, it has a quaternary structure. The amino acid sequence of the variable region decides its 3D shape, and the body can make millions of different variable regions, so there is an antibody to fit almost any antigen. At A-level, AQA expects you to know the structure of an antibody, including its variable and constant regions.

How do antibodies work?

An antibody doesn't usually kill a pathogen itself. It binds to the antigen, forming an antigen-antibody complex, and that triggers the pathogen's destruction in several ways:

• Agglutination: each antibody has two binding sites, so it can bind two pathogens at once. Many antibodies clump pathogens together, which stops them spreading and makes them easier to engulf.
• Marking for phagocytes: phagocytes have receptors that bind the antibodies' constant regions, so coated pathogens are recognised and engulfed (phagocytosis).
• Neutralising toxins: antibodies that bind to toxins, called antitoxins, block them from harming cells.
• Blocking viruses: antibodies that cover a virus's surface proteins can stop it entering cells.

Primary and secondary responses

The first time you meet a pathogen, it takes days for the right B lymphocytes to multiply and make enough antibody, so you may fall ill. Some of those lymphocytes become memory cells. If the same pathogen returns, memory cells respond much faster and make far more antibody, often destroying the pathogen before you feel ill. That is how you become immune, and why vaccines work: they trigger the first response safely, using a harmless form of the antigen.

Try sorting the body's defences, then press Mark to check them in our Human Physiology course.

Monoclonal antibodies

Monoclonal antibodies are identical antibodies made by a single clone of cells. They all bind to the same binding site on one particular antigen, so they can be used to find or target one specific substance or cell. César Milstein and Georges Köhler developed the method in 1975, and shared the 1984 Nobel Prize in Physiology or Medicine for it.

How are monoclonal antibodies produced?

This is the process AQA's GCSE Biology asks you to describe:

1. Stimulate: the chosen antigen goes into a mouse, which prompts its lymphocytes to make the matching antibody.
2. Collect: those lymphocytes are taken from the mouse. On their own they can make antibody but can't keep dividing.
3. Fuse: each lymphocyte is combined with a tumour cell, which divides endlessly. The fused cell is called a hybridoma, and it can both divide and make the antibody.
4. Clone: a single hybridoma cell making the right antibody is selected and cloned, giving many identical cells that all make the same antibody.
5. Purify: large amounts of the antibody are collected and purified.

Uses of monoclonal antibodies

• Pregnancy tests: monoclonal antibodies on the test stick bind to hCG, a hormone made in early pregnancy, and a coloured line appears if it is present.
• Diagnosis and lab tests: measuring levels of hormones and other chemicals in blood, or detecting pathogens.
• Research: antibodies joined to a fluorescent dye bind to one specific molecule, showing where it is in a cell or tissue.
• Treating disease: some cancer treatments use monoclonal antibodies that bind to cancer cells, either to help the immune system attack them or to carry a drug or radioactive substance straight to them, sparing other cells.

AQA also expects you to weigh up their pros and cons. Monoclonal antibodies are very specific, but when first used they caused more side effects than expected, and they are expensive to develop and make.

Common mistakes

• Mixing up antigens and antibodies. Antigens are on the pathogen. Antibodies are made by your B lymphocytes and bind to them.
• Saying antibodies kill pathogens directly. They mostly clump and mark pathogens so phagocytes can destroy them.
• Saying the antibody's shape is the same as the antigen's. It is complementary: it fits, like a lock and key.
• Saying any white blood cell makes antibodies. B lymphocytes (and the plasma cells they become) make them. Phagocytes engulf pathogens.
• Missing a step in monoclonal antibody production. Remember: stimulate, fuse with a tumour cell into a hybridoma, clone, purify.

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