Antigen (Ag), specifically recognized and bound by an antigen receptor on the surface of T/B lymphocytes, activates T/B cells, proliferates and differentiates, and produces an immune response product (sensitized lymphocytes or antibodies). Antigens have two important properties: immunogenicity and immunoreactivity. Immunogenicity refers to the ability of an antigen to induce a specific immune response in the body, producing antibodies and/or sensitizing lymphocytes. Immunoreactivity is the ability to specifically bind to a corresponding immune effector.

The basic properties of antigens are heterogeneous, macromolecular and specific:

(1) The heterogeneous property of the antigen refers to an antigenic substance that enters the body tissues, and must be different from the components of the body tissue cells. Antigen generally means that foreign substances entering the body can be substances between different species. They can be substances of the same kind or some isolated components from the body.

(2) The macromolecular property of the antigen means that the substance constituting the antigen is usually a macromolecular substance having a relative molecular mass of more than 10,000 KDa, and the larger the molecular weight, the stronger the antigenicity. Most proteins are good antigens. The antigenic substances are all macromolecular substances because the macromolecular substances can stay in the body for a long time, and have enough time to contact with the immune cells, causing the immune cells to react. If the foreign substance is a small molecule, it will be quickly excreted by the body, and there is no chance to contact the immune cells. If the macromolecular protein is hydrolyzed into a small molecule, the antigenicity is lost.

(3) The specificity of an antigen means that an antigen can only specifically bind to a corresponding antibody or effector T cell. The specificity of an antigen is determined by specific chemical groups on the surface of the molecule, and these chemical groups are called antigenic determinants. The antigen activates lymphocytes to cause an immune response by binding the antigenic determinant to the antigen receptor of the corresponding lymphocyte. The antigen also reacts with specific binding of the antigenic determinant to the corresponding antibody. Therefore, antigenic determinants are the material basis for the immune response and immune response.

Viral antigens are complete antigens that are both immunogenic and immune-reactive.

1. Adenovirus antigens

Adenovirusis a non-encapsulated particle with a diameter of 70-90 nm. It consists of 252 shells arranged in a squamous plane with a diameter of 7-9 nm. The capsid is a linear double-stranded DNA molecule containing approximately 4.7 kb and an inverted repeat of approximately 100 bp in length at each end. Since the 5' end of each DNA strand is covalently bound to a protein molecule having a molecular weight of 55X103Da, a cyclic structure of double-stranded DNA can occur.

There are currently 52 known human adenoviruses. The adenoviral genome is transcribed to produce mRNA, and there are at least five known transcription units: the E I region is located on the left side of the viral genome and subdivided into E IA and E IB, which are involved in cell transformation; the E II region encodes a DNA-binding protein involved in viral replication; the E III region encodes a glycoprotein that appears on the surface of the host cell; the EIV region is located at the right end of the ad2 genome and is regulated by the DNA binding protein encoded by the EII region; the fifth transcription unit synthesizes the ad2 protein IV in the middle of the viral infection. Adenovirus is a non-encapsulated spherical structure, and its virions are often arranged in a lattice in the infected nucleus. Each virion contains a 36 kb linear double-stranded DNA with a 100-600 bp antigen at each end. The inside of the inverted terminal re-peat (ITR) is a viral packaging signal, which is a cis-acting element required for viral packaging. The genome contains an early expressed E1~E4 gene associated with adenoviral replication and a lately expressed L1~L5 gene associated with adenoviral particle assembly. The linear double-stranded DNA and the core protein form a core of 60-65 nm in diameter and are wrapped in the capsid. The capsid is icosahedral and consists of 252 shells with a diameter of 8~10 nm. The shells are arranged on the surface of the triangle, 6 on each side, 240 of which are hexon (non-apical shell), and twelve are pentad bases (vertex shells). Each hexon is a homotrimer of a hexon protein. The hexon molecule of the trimer has a triangular apex and a pentahedral base, and the tower region consists of four rings, namely loop1, loop2, loop3 , loop4, the base contains two areas P1, P2. Epitopes on hexon are the criteria for diagnosing different serotypes, including the antigenic component of the mammalian adenovirus genus, which is the most sensitive site for virion selection. Each penta-substrate is bound to one (mammalian adenovirus) or two (avian adenovirus) long 9~77. 5 nm fiber protrusions, which are protruded from the shell surface by a penton protein. The apical end of the fiber forms the cephalic region, and the fibrosis is serum specific and contains a species-specific epitope responsible for hemagglutination in vitro.

The adenovirus contains 13% DNA and 87% protein, and the virion has a molecular weight of about 175 x 106. The viral genome is a linear double-stranded DNA containing approximately 35 kb to 36 kb, which is closely surrounded by protein VII and mu proteins, and functions as a histone-like. Another protein V links this DNA-protein complex and is linked to the viral capsid via protein VI. A specialized structure called a DNA terminal protein (pTP) complex (DNA-TPC) is covalently bonded to the 5' end of both strands and is closely related to adenovirus replication. The adenoviral genome has a 100 bp inverted terminal repeat (ITR) at each end, which is the origin of replication. On the 3' side of the left-end ITR, a packaging signal (ψ) of about 300 bp in length mediated the packaging of the adenoviral genome into the viral capsid. For adenovirus, only the about 0.5 kb sequence including the ITR at both ends and the packaging signal (ψ) is a cis-acting element, that is, it must be carried by the adenoviral vector itself, while the other 30 proteins can pass. The helper virus (or cell) is complemented.

Hexon protein is a major coat protein of adenoviruses. Adenoviruses capsids have three principal protein components: the hexon, the penton and the fiber. Hexon consists of three subunits together forming two major components of different morphological symmetry. A triangular top with three towers of density is superimposed on a more bulky pseudo hexagonal base. The symmetry of the top is in accord with the trimeric nature of hexon, but that of the base derives from the molecular function, which is to provide a densely packed impenetrable protective outer layer for the virion. Research indicates that the adenovirus hexon protein may be a potent adjuvant for activation of a cellular immune response.