An antigen is a substance that produces a particular immune response and causes a specific antibody or specially sensitized T cells, or both, to form. In nature, it's usually protein and sometimes polysaccharide. Although specific lymphocytes or antibodies recognize all antigens, only some antigens are capable of activating lymphocytes. Immunogens are called molecules that stimulate immune responses. The epitope is an immunologically active region of an immunogen (or antigen) that binds to lymphocyte antigen-specific membrane receptors or secreted antibodies.
Types of Antigen
Based on the order of their class (Origin):
1. Exogenous antigens
Such antigens enter the body or system and begin to circulate in the fluids of the body and the APCs trap them (Antigen processing cells, for example, dendritic cells, macrophages, etc.).
Phagocytosis mainly mediates the uptake of these exogenous antigens by APCs.
Examples: bacteria, fungi, viruses, etc.
Some antigens begin as exogenontigens, and become endogenous later on.
2. Endogenous antigens
These are the cells or sub-fragments or compounds of the body or the antigenic products that are formed.
The macrophages that are later accepted by the cytotoxic T-cells process the endogenous antigens.
Xenogenic (heterologous), autologous, and idiotypic or allogeneic (homologous) antigens are endogenous antigens.
Examples: antigens of the blood group, HLA (Histocompatibility Leukocyte antigen).
Autoantigen is usually a normal protein or protein complex (and sometimes DNA or RNA) that is recognized by a patient's immune system with a specific autoimmune disease.
These antigens should not be the target of the immune system under normal conditions, but the normal immunological tolerance of such an antigen has been lost in these patients, mainly due to genetic and environmental factors.
Nucleoproteins, nucleic acids, etc., are few examples.
Based on the Immune Response:
1. Immunogen or Complete Antigen
Possesses antigenic properties, i.e. they are capable of producing an immune response on their own.
The large weight of molecules (more than 10,000).
They may be polysaccharides or proteins.
2. Hapten or Incomplete Antigen
These are foreign substances, typically non-protein compounds.
They need the carrier molecule to serve as a full antigen as it is unable to cause an immune response by itself.
The carrier protein is a non-antigenic factor and helps to activate the immune response. Serum proteins such as albumin or globulin are common examples.
Low Weight in Molecules (Less than 10,000).
What Are Pathogens?
An organism that causes illness is a pathogen. Naturally, the body is full of microbes. However, if your immune system is compromised or if they manage to penetrate a usually sterile portion of your body, these microbes just cause a problem. Pathogens are distinct and, upon entering the body, can cause disease.
To flourish and live, all a pathogen requires is a host. When the pathogen sets itself up in the body of a host, it manages to inhibit the immune responses of the body and uses the resources of the body to reproduce before going out and spreading to a new host. Depending on the form, pathogens may be transmitted in a few ways.
The four major types of pathogens are bacteria, viruses, parasites, and fungi.
Viruses: Viruses consist of a fragment of genetic code, such as RNA or DNA, and are covered by a protein coating. Viruses enter host cells inside your body once you're infected. They then use host cell components to replicate, creating more viruses. These new viruses are released from the host cell after the replication cycle is complete. This normally damages the contaminated cells or kills them.
Bacteria: Microorganisms consisting of a single cell are bacteria. They are very diverse, have a range of shapes and characteristics, and are able to live in any environment, including your body and yourself. Bacteria don't just cause infections. The ones that can cause infections are referred to as pathogenic bacteria.
Fungi: On Earth, there are millions of different fungal species. It is believed that sickness is caused by just 300 sources or so. In the world, fungi can be found just about anywhere, including outdoors, indoors, and on human skin. When they overgrow, they cause infection. Fungi cells have a nucleus and other components that are covered by a membrane and a thick cell wall. Their structure makes their elimination difficult.
Parasites: Parasites are organisms small animals that live in or on a host and survive from or at the expense of the host. While parasitic infections in tropical and subtropical regions are more common, they can occur anywhere.
In humans, three primary types of parasites may cause illness. They include:
Protozoa, that are single-celled species in your body that have the ability to survive and multiply.
Helminths, multi-celled and larger organisms which are commonly termed worms. They can live in or out of your body.
Ectoparasites, which are multi-celled organisms that, like certain insects such as ticks and mosquitoes, live on or feed off your skin.
Difference Between Antigen And Pathogen
It can be painful to have a fever and inflammation, but they're signs of your body doing its job. Fever activates white blood cells, boosts metabolism, and stops the multiplication of certain microorganisms within the body.
Physical activity such as exercise and running can help in protecting you against the common cold. It benefits by strengthening the immune system and minimizing the stress hormone levels.