Antigens Involved in Adaptive Immunity

Complete antigens, or simply antigens, are large, complex foreign molecules, mainly proteins, that initiate an immune response.

They possess two key characteristics — immunogenicity, the ability to stimulate specific lymphocyte proliferation, and reactivity, which allows them to interact with activated immune cells and antibodies.

Lymphocytes recognize specific regions of an antigen known as antigenic determinants or epitopes.

Most antigens have multiple epitopes, each capable of inducing a specific antibody or T-cell activation.

Generally, small molecules such as peptides and nucleotides have reactivity but lack immunogenicity, so they are termed haptens or incomplete antigens. They can prompt an immune response when attached to a larger carrier molecule.

For example, a small lipid toxin in poison ivy or drugs like penicillin can form immunogenic complexes with body proteins.

To distinguish themselves from non-self cells, host cells have two classes of self-antigens called the major histocompatibility complex, or MHC.

All nucleated body cells carry the MHC-I glycoproteins, while the MHC-II glycoproteins are found in the antigen-presenting cells.