The formation of an antigen-antibody complex, leading to the destruction…

The idea

An antibody's binding site has a tertiary structure complementary to a specific antigen, so the two bind to form an antigen-antibody complex. Each antibody has two binding sites, so one antibody can bind antigens on two separate bacterial cells. With many antibodies present, bacteria are cross-linked and pulled into clumps: this is agglutination. Clumping prevents the bacteria spreading and lets a single phagocyte engulf many bacteria at once. The antibody's constant region also binds receptors on the phagocyte's membrane, so the bacterium is engulfed by phagocytosis and hydrolysed by lysosomes. The specification limits destruction of the antigen here to agglutination and phagocytosis of bacterial cells, so keep the answer to those two mechanisms.

What earns the marks

• The antibody binding site being complementary to the antigen, so an antigen-antibody complex forms.
• Each antibody having two binding sites, so one antibody binds antigens on two different bacterial cells.
• Agglutination, described as the clumping together of bacteria by antibodies.
• Phagocytes engulfing the clump, so many bacteria are destroyed at once.
• Explaining why clumping helps: one phagocyte can engulf many bacteria, and the bacteria cannot spread as easily.

Common mistakes and how to put them right

• Saying the antibody dissolves, digests or kills the bacterium itself.

  Put it right: Antibodies do not destroy bacteria directly, phagocytes do.

• Describing agglutination as antibodies sticking to each other rather than binding antigens on separate cells.

• Explaining agglutination and then stopping, when the specification pairs it with phagocytosis of the clumped bacteria.

• Writing 'the antibody destroys the antigen' with no mechanism at all.

• Forgetting that an antibody with only one binding site could not clump anything, because cross-linking needs two.

Exam tips

• Two binding sites is the sentence that makes agglutination work; include it every time.
• Say why clumping helps, one phagocyte engulfs many bacteria, rather than leaving the examiner to infer it.
• Stick to agglutination and phagocytosis of bacterial cells; the specification limits destruction of the antigen to these two mechanisms.