What antibodies are
An antibody is a Y-shaped protein produced by B lymphocytes (as plasma cells). Each antibody is specific: its binding site fits only one type of antigen, rather like a key fitting one lock. When it binds, it forms an antigen-antibody complex. Antibodies are also called immunoglobulins and are carried in the blood plasma and other body fluids, where they patrol for matching antigens.
Key idea
Specificity: one antibody recognises one antigen. This is why immunity to measles does not protect against chickenpox.
How antibodies destroy pathogens
Antibodies act in several ways:
- Agglutination — antibodies clump many pathogens together, making them easier for phagocytes to engulf and stopping them spreading.
- Neutralisation — antibodies bind to and block toxins or the surfaces pathogens use to attach to cells, making them harmless.
- Precipitation — soluble antigens are bound and made insoluble so they can be removed.
- Opsonisation — antibodies coat a pathogen, marking it so phagocytes recognise and engulf it more easily.
- Lysis — with the help of complement proteins, antibodies can cause the pathogen's membrane to burst.
Working with other defences
Antibodies rarely act alone. By clumping and marking pathogens they make the work of phagocytes far more effective, linking the specific and non-specific defences together. In other words, antibodies do not usually kill pathogens by themselves; instead they disable them and label them so that phagocytes and complement proteins can finish the job quickly.
After an infection, some B lymphocytes remain as memory cells. These can make the same antibody very quickly if the same antigen returns, which is the basis of long-term immunity. This teamwork makes the whole immune response both powerful and highly precise.
Example
Antibodies against tetanus bind to and neutralise the bacterial toxin, so it can no longer damage nerve cells even before the bacteria are cleared.