Adjuvant: Meaning, Examples and Activities

In this article we will discuss about:- 1. Meaning of Adjuvants 2. Examples of Adjuvants 3. Activities.

Meaning of Adjuvants:

A substance, distinct from antigen that when mixed with an antigen and injected with it, enhances the immunogenicity of that antigen is called adjuvant (Latin: adjuvare means to help). If the amount of administered antigen is small or when antigen has low immunogenicity, adjuvants are used with those antigen to boost the immune response.

Examples of Adjuvants:

According to the nature, adjuvants may be grouped into three categories:

1. Depot:

Alum (Aluminium potassium sulfate), aluminium hydroxide or phosphate, Freund’s incomplete adjuvant (water, mineral oil, mannide monooleate) etc. These adjuvants prolong the persistence of antigen. When the antigen is mixed with the alum, the salt precipitates the antigen.

Injection of this mixture results in a slower release of antigen from the site of injection so that the effective time of exposure to the antigen increases. Other depot adjuvants also prolong the persistence of antigen within the body.

2. Bacterial:

Freund’s complete adjuvant. It contains heat-killed Mycobacteria in water-in-oil emulsion. The muramyl dipeptide component of bacterial cell wall activates the macrophages resulting in enhanced expre­ssion of class II MHC molecules, B7 molecules and secretion of more cytokines (IL-1). All such co-stimulators stimulate TH cell activity.

Both alum and Freund’s adjuvants cause a local, chronic inflammatory response at the site of application. It attracts both phagocytes and lymphocytes that often result in the formation of a dense, red mass of cells called granuloma. Activated macrophages in the granuloma in turn, activate the TH cell-response.

3. Chemical:

Silica particles, berrilium sulfate, calcium alginate etc. Other adjuvants, for example, synthetic polyribonucleotides and plasmid DNA have intrinsic adjuvant-like activities. These can stimulate nonspecific proliferation of lympho­cytes and, thus, increase antigen-induced clonal selection of lymphocytes.

Activities of Adjuvants:

i. Most adjuvants elicit innate immune responses, with increased expression of co-stimulators and production of cytokines that stimulate T cell growth and differen­tiation.

ii. Adjuvants induce local inflammation and thus stimulate the influx of APCs to sites of antigen exposure.

iii. Adjuvants may also act on APCs to prolong the persistence of peptide-MHC complexes on the cell surface.

Adjuvants, thus promote T cell activation by several mechanisms. Therefore, to induce a T cell response to a protein antigen in a vaccine or experiment, the antigen must be administered with adjuvants.

In fact, most potent adjuvants are microbial products which can’t be used in human because of the pathologic inflammation that microbial products elicit. However, attempts are ongoing to develop adjuvants for clinical use, mainly to maximise the immuno­genicity of vaccines.