Difference Between Antigen And Pathogen: Structure & Types

Antigen and pathogen play an important role in the immunity of our bodies. They also play a crucial function in microbiology. An antigen is a chemical that can activate an immune response, whereas a pathogen is a disease-causing bacterium. Bacteria, viruses, and different microbes can be pathogens. An antigen is a protein, polysaccharide, or lipid found on a pathogen's surface. Exogenous antigens are antigens that are found on pathogens. Endogenous antigens, autoantigens, and neoantigens are examples of other forms of antigens. Pathogens use specialised strategies to escape immune responses and survive and multiply inside the host. In this article, we will explore differences, properties, types and structures in detail.

Key Takeaways: Antigen, Pathogen, Antibody, Bacteria, Immunity, Vaccination, Immune System, Immunization

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What is Antigen?

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Antigen is a chemical that can activate lymphocytes, the body's infection-fighting white blood cells, and stimulate an immunological response. Antigens are divided into foreign antigens (heteroantigens) and autoantigens (self-antigens).

• Foreign antigens are those that come from outside the body.
• Autoantigens, on the other hand, are created by the body itself.

Normal physiological substances trigger an immune response to produce autoantibodies, and the body can tell the difference between self and non-self. However, when people have autoimmune diseases, normal physiological substances trigger an immune response, resulting in the production of autoantibodies.

Properties of Antigen

Antigens have a variety of characteristics that are as follows:

• The ability to recognise and distinguish between self and non-self molecules is conferred by foreignness. Self-molecules do not elicit an immune response, but non-self molecules do.
• With a molecular weight of 10,000, immunogenic molecules are huge and complicated. Weak immunogens are weakly immunogenic, whereas non-immunogenic immunogens, such as amino acids, are very tiny. When haptens are coupled to a carrier protein, they become immunogenic.
• Antigens must have a particular level of chemical complexity and a structure with a large number of antigenic determinants (epitopes) in order to elicit an immune response, as opposed to small simple peptides with only a few epitopes.
• Two strains of the same type of human being can react to the same antigen in different ways due to differences in the genetic makeup of genes involved, such as MHC alleles, which are generally distinct.
• Because the degree of immune response is greatly dependent on the amount of antigen provided, the immune response can be improved by changing the dosage, route of delivery, and time of administration.

Antigen

Types of Antigen

Antigens are classified as:

1. Based on their origin

• Exogenous Antigens

Exogenous antigens are those that originate outside of the host's body and are hence foreign to the host. These antigens may enter the body through inhalation, ingestion, or injection, and then circulate through bodily fluids throughout the body.

Follicular Dendritic Cells

• Endogenous Antigens

Antigens that arise within the body of the host during metabolism or as a result of intracellular viral or bacterial infection are known as endogenous antigens. The cells of the body, as well as fragments, chemicals, and antigenic products of metabolism, are examples of endogenous antigens.

• Autoantigens

Autoantigens are proteins or protein complexes that are attacked by the immune system of the host, causing autoimmune illness. Autoantigens can be fatal to the host because the immune system should not attack the body's own cells. Due to hereditary and environmental factors, immune tolerance to such antigens is lost.

• Tumor Antigens (Neoantigens)

Major Histocompatibility Complex (MHC) I and II present tumour antigens or neoantigens on the surface of tumour cells. During the malignant transformation of normal cells, antigens are created as a result of a tumour-specific mutation. Because tumour cells evolve mechanisms to circumvent antigen presentation and immune defence, these antigens seldom elicit an immunological response.

• Native Antigens

Native antigens are antigens that have not been processed by any antigen-presenting cells (APC), hence immune cells such as T-cells are unable to bind to them whereas B-cells can be triggered by such antigens without any processing.

2. Based on immune response

• Complete antigens/Immunogens

Antigens that trigger a specific immune response are known as complete antigens or immunogens. Without any carrier particles, these antigens can elicit an immunological response.

• Incomplete antigens/Haptens

Antigens that are incomplete or haptens are antigens that cannot elicit an immune response on their own. These are usually non-protein compounds that require the presence of a carrier molecule in order to function as a full antigen.

Read More: Difference between Antigen and Antibody

Structure of Antigen

Antigen can bind to the antigen-binding site of an antibody.

• Antibodies distinguish between distinct antigens based on the unique molecular structures found on the antigen's surface.
• Proteins and polysaccharides make up the majority of antigens. Coats, capsules, flagella, poisons, and fimbriae of bacteria, viruses, and other microbes are examples. In addition, secretions and other similar substances can behave as antigens.
• These microbes' lipids and nucleic acids are only antigenic when coupled with proteins or polysaccharides.
• Antigens can have a variety of structures based on the antigen's composition, size, and immunogenicity.
• Epitope or antigenic determinant is a structural component found in all immunogenic antigens.
• Various antigens have different numbers of epitopes, which dictates how many antibodies a particular antigen may bind to.
• Antigens have diverse structural components of interaction, which defines which antibody classes they bind to.

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What is Pathogen?

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Pathogen is a disease-causing organism. These microorganisms only become an issue if your immune system is compromised or if they enter a typically sterile portion of your body. A pathogen requires a host to flourish and survive. Once within a host's body, the pathogen manages to dodge the body's immune responses and replicates using the body's resources before escaping and spreading to a new host. Depending on the pathogen, it can spread in a variety of ways including skin contact, body fluids, airborne particles, contact with faeces, and touching a surface touched by a sick individual.

Transmission of Human Pathogen

Properties of Pathogen

Pathogens' fundamental goal is to survive and multiply in their host. Their goal is to find a nutrient-rich bodily component that is suitable for them in order to survive. Pathogens are highly good at studying a cell's process and adapting to exploit its host organisms.

Types of Pathogen

Pathogens are in various forms, but the four most common are viruses, bacteria, fungi, and parasites.

Viruses

Viruses are made up of a bit of genetic information (DNA or RNA) that is protected by a protein coating. They infect host cells in your body once you've been infected and then multiply using the components of the host cell producing more viruses. These new viruses are expelled from the host cell once the replication cycle is complete. The diseased cells are frequently damaged or destroyed as a result of this.

Some viruses can go latent for a while before reactivating. When this happens, a person looks to have healed from a viral illness but then becomes ill again. Antibiotics are ineffective for the treatment of viral illnesses because they do not kill viruses. Depending on the virus, antiviral drugs may be employed.

Read More: Infectious Diseases- Definition, Types, Symptoms and Preventions

Bacteria

Bacteria are microorganisms that are made up of only one cell. They come in a wide range of forms and sizes, and they can live in just about any environment, including within and on your body. Infections aren't caused by all bacteria. Pathogenic bacteria are microorganisms that can cause disease. When your immune system is weakened by a virus, you may be more susceptible to bacterial infections.

A virus-caused illness allows microorganisms that are typically innocuous to become harmful. Antibiotics are drugs that are used to treat infections caused by bacteria. Antibiotic resistance has developed in several bacterium strains, making treatment difficult.

Fungi

On the planet, there are millions of distinct fungus species. Only about 300 are known to cause illness. Fungi are found almost everywhere, including indoors, outdoors, and on people's skin. When they outgrow their confines, they spread infection.

A nucleus and other components of fungi cells are protected by a membrane and a thick cell wall. They are more difficult to destroy because of their structure.

Parasites

These microorganisms live in or on a host and feed on or at the expense of the host. Despite the fact that parasite diseases are more common in tropical and subtropical areas, they can happen anywhere. Human disease can be caused by three types of parasites:

• Single-celled organisms that may live and multiply in your body are called protozoa.
• Helminths, sometimes known as worms, are bigger, multi-celled organisms that can live within or outside your body.
• Some insects, such as ticks and mosquitos, are ectoparasites, which are multicellular organisms that live on or feed off your skin.

They spread through contaminated soil, water, food, and blood, as well as sexual contact and insect bites.

Structure and Locomotor Organelles of Protozoa

Read More: Difference between Pathogens and Parasites

Structure of Pathogen

We will take the example of Bacteria to understand the pathogen structure.

Bacteria are recognised for having a basic body structure and are single-celled microorganisms lacking a nucleus and other cell organelles, classifying them as prokaryotic creatures. They are also incredibly adaptable species, surviving in the most hostile of environments.

Extremophiles are organisms that live in extreme environments. Extremophiles are further divided into different categories based on the habitats they live in: Thermophiles, Acidophiles, Alkaliphiles, Osmophiles, Barophiles and Cryophiles.

Structure of Bacteria

The protective cell wall of bacteria, which is made up of a specific protein called peptidoglycan, is another remarkable trait. Except in the cell walls of bacteria, this protein isn't found anywhere else in nature. However, only a few species lack this cell wall, while others have a third layer of protection known as the capsule.

One or more flagella or pili are attached to the outer layer, which serves as a locomotory organ. Pili can also assist germs in attaching to the cells of the host. Except for ribosomes, they lack any cell organelle found in animal or plant cells. Protein synthesis takes place on ribosomes. They also have an extra circular DNA called plasmid in addition to this DNA. Some bacteria strains are resistant to antibiotics because of these plasmids.

Read More: Difference between Innate Immunity and Adaptive Immunity

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Difference between Antigen and Pathogen

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The differences between antigen and pathogen are:

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Things to Remember

• Pathogens can make us sick, but when we're healthy, our bodies can fight off pathogens and the illnesses they cause.
• Antigen is a foreign substance or toxin that triggers an immune response in the body, particularly antibody production.
• Self-antigens are antigens located in the body's own cells, and the immune system generally does not assault them.
• Antibodies are proteins produced by the body to aid in the battle against antigens, which are foreign substances.
• Antibodies adhere to or bind to antigens, rendering them inactive.
• The immune system protects the body from infections. It fights germs and keeps us healthy. When the body detects foreign substances (known as antigens), the immune system strives to identify and eliminate the antigens.
• Immunity refers to your body's ability to recognize pathogens and prevent sickness.
• Innate immunity refers to the immune system present at birth, whereas acquired immunity refers to a disease resistance that develops with time.

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