Question

Antigens are found
(A) Inside the cytoplasm
(B) Inside the nucleus
(C) On nuclear envelope
(D) On cell surface

Answer 1

Antigen or immunogen is any foreign substance, toxin, particle or pathogen which induces the immune system of the body to produce cells and antibodies to dispose of the same. Molecular mass of antigen is 8000 Daltons or more. Substances with smaller molecular mass can also produce an immune response provided they first combine with an antigenic substance. They are called Haptens, e.g., drugs, animal dandruff, dust components, industrial chemicals, Certain poisons. Haptens are also called incomplete antigens while others which elicit immune response are called complete antigens.

Complete answer:
Antigenic substances are generally proteins and large polysaccharides. This includes parts (cell wall, flagella, coat, capsule, fimbriae and toxins) of viruses, bacteria and other microorganisms. When lipids and nucleic acids combine with protein and polysaccharides then only they can be antigenic.
Antigens are categorized based on their origin. Diversity among antigens is analogous to the diversity of the diseases that the immune system works to overcome.
Main classes of antigens involved in immune system activation are:
1. Exogenous antigens: The antigens that entered the body from the outside. For example by ingestion, inhalation or injection. The most common kinds of antigen are exogenous antigens. They include pollen or food that may cause allergies as well as molecular components of bacteria and other pathogens that could cause an infection.
2. Endogenous antigens: Endogenous antigens are the antigens that have been generated within previously normal cells as a result of normal cell metabolism or because of intracellular bacterial and viral infection (they change cells from inside in order to reproduce). The fragments are then presented on the surface of the infected cells in the complex with MHC class I molecules.
3. Autoantigens: Autoantigens are the antigens that stimulate autoantibodies in the Organism that produces it. These are self antigens involved in autoimmune disease pathogenesis.
Antigens are present on the surface of the cell.

So, the correct option is (D).

Note:
Pathogens function as antigens because they possess either antigenic material on the surface or produce antigenic toxins. Sites present over the surface of the antigen which are recognized by T- and B- lymphocytes as well as antibodies are called antigenic determinants or epitopes. The region of antibodies and lymphocytes which function as receptors for epitopes are called paratopes. Epitopes or antigenic determinants have a regular recurring molecular group which provides stereochemical characteristics. It is not a polymer of single amino acid or single sugar but a copolymer or heteropolymer. Because of it proteins are the most common antigens.