Actions of Antibodies

Antibody structure and specificity

An antibody is a Y-shaped protein produced by B-lymphocytes (specifically plasma cells) in response to a particular antigen, a marker molecule on the surface of a pathogen. Each antibody has two identical antigen-binding sites whose shape is complementary to one specific antigen, much like a lock fits only one key.

This specificity means an antibody made against one pathogen will not usually bind to a different, unrelated pathogen.

How antibodies act on pathogens

• Agglutination: because each antibody has two binding sites, it can link two pathogens together at once; antibodies acting together clump large numbers of pathogens, immobilising them and making it easier for phagocytes to engulf a whole clump at a time.
• Neutralisation: antibodies bind to toxins released by pathogens, or to the surface proteins pathogens use to attach to and invade body cells, blocking their harmful effect.
• Opsonisation: antibodies coat the surface of a pathogen, acting as a marker (or 'tag') that makes it easier for phagocytes to recognise and engulf the pathogen.
• Lysis: some antibodies, working with other blood proteins, damage a pathogen's cell membrane so severely that it bursts and is destroyed.

From first exposure to lasting protection

The first time the body meets a particular antigen, it takes days to weeks to produce enough matching antibody, because the correct B-lymphocytes must first be identified and multiply, this is the primary response. Once the infection is cleared, some B-lymphocytes remain in the body as memory cells.

If the same antigen is met again, these memory cells allow a much faster and larger secondary response, often clearing the pathogen before symptoms even appear, this fast recall is the basis of long-term immunity.

How it is examined

Questions on this standard often ask you to match a named action (agglutination, neutralisation, opsonisation or lysis) to its description or diagram, to explain why an antibody only fits one antigen using the lock-and-key idea, or to interpret a graph comparing antibody concentration during a primary and a secondary immune response.

Worked exam-style question

Question. A graph shows the concentration of antibody in a person's blood after two injections of the same vaccine, given a few weeks apart. After the first injection the antibody level rose slowly to a low peak, then fell.

After the second injection it rose much faster to a far higher peak. (a) Name the type of white blood cell that produces antibodies.

(b) Explain why the response to the first injection was slow. (c) Explain why the response to the second injection was faster and larger.

(d) State one way an antibody, once made, disables a pathogen.

Model answer. (a) Antibodies are produced by B-lymphocytes (plasma cells). (b) The first injection gives a primary response: the few B-lymphocytes with a receptor matching the antigen must first be selected and divide before enough antibody can be made, which takes time.

(c) After the first response, some B-lymphocytes remain as memory cells; on the second exposure they recognise the same antigen at once and quickly divide and produce antibody, giving a faster, larger secondary response. (d) Any one: agglutination (clumping pathogens), neutralisation (blocking toxins or attachment), opsonisation (marking for phagocytes) or lysis (bursting the pathogen).

Practice question

Try this. A person is given ready-made antibodies by injection (passive immunity). Explain why this gives protection quickly but does not produce long-lasting immunity.

Exam tip

Key terms

These terms recur in immunity questions, use them precisely:

• Antibody, the Y-shaped protein that binds one specific antigen.
• Antigen, the marker on a pathogen that the antibody recognises.
• Pathogen, the disease-causing organism the response targets.
• Immunity, lasting protection built from memory cells.
• Vaccine, a harmless dose of antigen that triggers a primary response.

Source:SRC-DSKP-EN

Frequently asked questions

More for Immunity in Humans