Difference Between Antigen and Pathogen

Main Difference – Antigen vs Pathogen

Antigen and pathogen are two factors involved in triggering immune responses in animals. The main difference between antigen and pathogen is that antigen is a molecule that can trigger an immune response whereas pathogen is a disease causing microorganism. Pathogens can be a bacterium, virus or other microorganisms. An antigen can be a protein, polysaccharide or lipid on the surface of a pathogen. The antigens present on the pathogens are called exogenous antigens. Other types of antigens include endogenous antigens, autoantigens, and neoantigens. Pathogens have specific mechanisms to survive and multiply inside the host by avoiding the immune responses.

Key Areas Covered

1. What is an Antigen
      – Definition, Characteristics, Types
2. What is a Pathogen
      – Definition, Characteristics, Types
3. What are the Similarities Between Antigen and Pathogen
      – Outline of Common Features
4. What is the Difference Between Antigen and Pathogen
      – Comparison of Key Differences

Key Terms: Antibody, Antigen, Autoantigens, Disease, Endogenous Antigens, Epitope, Exogenous Antigens, Immune Response, Neoantigens, Pathogen

What is an Antigen

An antigen refers to any substance that is recognized by the body as foreign and triggers an immune response. Antigens can be proteins, polysaccharides, lipids or nucleic acids. An epitope or antigen determinant is the part of an antigen that gets attached to the antibody. An antibody is a glycoprotein molecule produced in response to a specific antigen. Antibodies are produced by the plasma cells in blood after identifying a foreign substance in the body. The four major types of antigens are exogenous antigens, endogenous antigens, autoantigens, and neoantigens.

Figure 1: Antigen and Epitopes

The exogenous antigens are present on the surface of the pathogens or microorganisms that invade the body. The endogenous antigens are the metabolic products of the pathogens produced inside the body. The autoantigens are the molecules or cells in the body, which are mistakenly recognized as non-self by the immune system. This type of recognitions can cause autoimmune diseases, destroying self-tissues and organs in the body. The neoantigens are the molecules expressed on the surface of the cells infected by oncogenic viruses. Different epitopes on an antigen are shown in figure 1.

What is a Pathogen

A pathogen refers to an agent that cause diseases or illnesses in the host. Pathogens can be microorganisms such as bacteria, virus, fungi, protozoa, algae, and parasites. Typically, microorganisms can be found in or on the human body. These microorganisms usually do not cause any diseases. They are called natural flora. The natural flora live either on the skin, in the mouth, intestine or vagina. However, there is another type of microorganisms which can cause diseases and illnesses to the host. They are called pathogens. Generally, pathogens are recognized by the immune system of the host by identifying various antigens on the surface of the pathogens. The recognition of a foreign antigen triggers an immune response which destroys the pathogen. Antibodies are produced in response to a specific pathogen in the body, and these antibodies bind to the specific antigens to neutralize it. The binding of antibodies to the pathogen may either immobilize the pathogen or lyse the pathogen by letting it be recognized by phagocytic cells in the immune system. Pathogens can also be destroyed through complement reactions by binding complement proteins to the pathogen.

Figure 2: Eucalyptus Leaf and Unknown Pathogen

In addition, some pathogens have developed specialized mechanisms for the survival and multiplication inside the host. They can avoid host’s innate and adaptive immune responses. The signs and symptoms of a disease can arise either by the pathogen or the response of the host.

Similarities Between Antigen and Pathogen

Both antigen and pathogen trigger an immune response in the host.
Both antigen and pathogen can be considered as immunogens.
Both antigen and pathogen can cause diseases in the host.

Definition

Antigen: An antigen is a molecule capable of inducing an immune response in the host organism.

Pathogen: A pathogen is an agent that can cause diseases or illnesses in the host.

Correlation

Antigen: Antigens can be a molecule on the surface of the pathogen, triggering an immune response.

Pathogen: Pathogen can cause diseases.

Organization Level

Antigen: Antigens are can be proteins, polysaccharides, lipids or nucleic acids.

Pathogen: Pathogens are organisms.

Types

Antigen: Antigens can be exogenous antigens, endogenous antigens, autoantigens, or neoantigens.

Pathogen: Pathogens can be bacteria, virus, fungi, protozoa, or parasites.

Conclusion

Antigen and pathogen are two types of immunogens that can trigger an immune response in host organisms. An antigen refers to a substance that triggers an immune response in a host. Antigens can be proteins, polysaccharides or lipids. They are sometimes present on the surface of pathogens that invade the host. A pathogen is a disease-causing agent to the host. Pathogens are organisms while antigens are molecules. Therefore, the main difference between antigen and pathogen is the organization level of each immunogen.

Reference:

1.“ Antigen.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 7 June 2016, Available here. Accessed 31 Aug. 2017.
2. Alberts, Bruce. “Introduction to Pathogens.” Molecular Biology of the Cell. 4th edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here. Accessed 31 Aug. 2017.

Image Courtesy:

1. “Figure 42 02 03” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Eucalyptus Leaf and Unknown Pathogen” by Forest Starr and Kim Starr (CC BY 2.0) via Flickr