Data expansion:

Antigen (abbreviated as Ag) refers to the substance that can cause antibody production, and it is any substance that can induce immune response. Foreign molecules can be recognized by immunoglobulins on B cells, or treated by antigen-presenting cells and combined with major histocompatibility complexes to reactivate T cells and trigger a sustained immune response.

Antigen (Ag) refers to all substances that can induce immune response. That is, it can be specifically recognized and bound by antigen receptor (TCR/BCR) on the surface of T/B lymphocytes, activate T/B cells, make them proliferate and differentiate, produce immune response products (sensitized lymphocytes or antibodies), and can specifically bind with corresponding products in vitro and in vivo.

Therefore, antigenic substances have two important characteristics: immunogenicity and immunoreactivity. Immunogenicity refers to the ability of antigen to induce specific immune response, produce antibodies and/or sensitize lymphocytes; Immunoreactivity refers to the ability to react specifically with corresponding immune effector substances (antibodies or sensitized lymphocytes) in vivo and in vitro.

According to the nature of antigen, it can be divided into two categories: complete antigen and incomplete antigen. Complete antigen is abbreviated as antigen. It is a substance with immunogenicity and immunoreactivity. For example, most protein, bacteria, viruses and bacterial exotoxins are complete antigens.

Incomplete antigen, that is, hapten, is a substance with only immunoreactivity and no immunogenicity, so it is also called incomplete antigen. After hapten is combined with protein carrier, immunogenicity is obtained. It can also be divided into complex haptens and simple haptens.

Compound hapten has no immunogenicity, but only immunoreactivity, such as most polysaccharides (such as pneumococcal capsular polysaccharide) and all lipids. Simple haptens are neither immunogenic nor immunoreactive, but they can prevent antibodies from binding to corresponding antigens or complex haptens. For example, the hydrolysate of pneumococcal capsular polysaccharide.

According to whether T cells are needed to help antigens stimulate B cells to produce antibodies, they can be divided into thymus dependent antigen (TD-Ag) and Thymus Independent Antigen (TI-Ag). TD-Ag is an antigenic substance, which needs the assistance of T cells and the participation of macrophages to activate B cells to produce antibodies.