Question

Explain various ways antibodies act in the body to provide immunity.

Answer 1

Antibodies provide immunity by forming antigen-antibody complexes with the specific antigens. They do so by three main mechanisms:

(i) Agglutination: An antibody has two antigen-binding sites and can link two antigens/microbes together, causing them to clump. The clumped microbes are more easily recognised and engulfed by phagocytes. 

(ii) Opsonisation: Antibodies coat the surface of the microorganisms (acting as opsonins), making them more readily recognised and engulfed by macrophages and neutrophils. 

(iii) Neutralisation: Antibodies bind to and neutralise the toxins produced by pathogens (e.g., diphtheria, tetanus toxins), and also neutralise viruses so that they can no longer infect host cells.

In addition:

(iv) Activation of complement: Bound antibodies activate the complement system which forms pores in the microbial membrane and causes its lysis. 

(v) Stimulation of phagocytes: IgG stimulates phagocytes to engulf and digest the pathogens. 

(vi) Prevention of pathogen binding: Antibodies block the attachment sites on the pathogen so that it cannot attach to host cells.

Thus antibodies act by agglutination, opsonisation and neutralisation of microbes and their toxins, and by activation of the complement system and phagocytes.