Clinical application of antigen and antibody

I don't know if you have checked the antigen and antibody indicators when you went to the hospital for a physical examination, but even if you have, you probably don't know what they mean. When certain diseases occur in the body, doctors will ask patients to check the antigens and antibodies in the body. So what is the application of antigen and antibody testing in clinical medicine? Let’s learn about it together.

The purpose of specific antibody detection is first to assist clinical diagnosis. In some diseases, it is also an indicator for observing the efficacy and prognosis. The detection of specific antibodies also has special and important significance in the observation of vaccination effects and in epidemiological surveys of infectious diseases.

Generally speaking, it would be ideal to diagnose infectious diseases if pathogens could be directly detected from specimens. However, since some pathogens require high growth conditions, take a long time to grow, and have a low positive detection rate, this poses certain difficulties for clinical diagnosis. The detection of specific antibodies can make up for the above deficiencies to a certain extent. The gradual development in recent years of using immunological methods to determine pathogenic antigens in specimens or using molecular biology techniques to determine infectious factors is undoubtedly a huge step forward in the early diagnosis of infectious diseases. Despite this, it cannot completely replace the detection of specific antibodies, such as human immunodeficiency virus (HIV), the determination of its antibodies is still an important basis for diagnosing AIDS.

Antigen (Ag) is any substance that can induce an immune response. Foreign molecules can be recognized by immunoglobulins on B cells or processed by antigen-presenting cells and combined with major histocompatibility complexes to form complexes that reactivate T cells and trigger a continuous immune response.

According to the nature of antigens, they are divided into two categories: complete antigens and incomplete antigens. Complete antigen (complete antigen) is referred to as antigen. It is a type of substance that is both immunogenic and immunoreactive. For example, most proteins, bacteria, viruses, bacterial exotoxins, etc. are complete antigens.

An incomplete antigen, also known as a hapten, is a substance that only has immunoreactivity but no immunogenicity, so it is also called an incomplete antigen. When the hapten is combined with a protein carrier, it acquires immunogenicity. It can be divided into complex haptens and simple haptens. Complex haptens are not immunogenic, but only immunoreactive, such as most polysaccharides (such as the capsular polysaccharide of pneumococcus) and all lipids; simple haptens are neither immunogenic nor immunoreactive, but can prevent antibodies from binding to corresponding antigens or complex haptens. Such as the hydrolysis products of pneumococcal capsular polysaccharides.

Depending on whether antigen-stimulated B cells need T cell assistance to produce antibodies, they can be divided into thymus-dependent antigens (TD-Ag) and thymus-independent antigens (TI-Ag). TD-Ag refers to an antigenic substance that requires T cell assistance and macrophage participation to activate B cells to produce antibodies. Characteristics of TD antigen immune response: can induce humoral immune response as well as cellular immune response; produce various types of antibodies such as IgG; can induce immune memory. TI-Ag refers to an antigen that can directly stimulate B cells to produce antibodies without the help of T cells. Its characteristics are: it can only cause humoral immune response; it can only produce IgM antibodies; it has no immune memory

According to their sources, antigens can be divided into:

(1) Xenoantigens: antigens between different races such as pathogenic microorganisms and toxoids;

(2) Alloantigens: antigens that exist between different individuals of the same race, such as HLA, ABO blood group antigens, Rh antigens, MHC, etc.;

(3) Autoantigens: Self-components, divided into hidden autoantigens, altered autoantigens, etc., such as eye lens proteins;

(4) Heterophilic antigens: also known as Forssman antigens, they are common antigens that exist on the surfaces of different species without species specificity. They can exist in animals, plants, microorganisms and humans. For example, the common antigens that hemolytic streptococci have in the human endocardium or glomerular basement membrane are heterophilic antigens.

In addition, antigens can be divided into:

(1) Endogenous antigens: antigens produced by the target cells of immune effector cells themselves;

(2) Exogenous antigens: refers to antigens that are not produced by the APC itself. As well as natural antigens (natural Ag), artificial antigens (artificial Ag), synthetic antigens (synthetic Ag), etc.