After the entry of an Antigen, what happens?

What is Antigen?

By immunological definition, we can say that “an antigen (Ag) is any chemical substance that can stimulate a body’s immune system to produce antibodies”. As they are able to stimulate the body’s immune response thus they are also called immunogens. Basically, they are large protein molecules that are normally found on tumor cells, pathogens like bacteria, viruses, fungi, and even in our normal body cells. Sometimes particulate matter like dust or pollen grains can act as antigens. Other than large molecules of proteins they can be just amino-acid chains or peptides, polysaccharides, nucleic acids, and lipids. They can be their own body-based origin or external environmental origin. In the case of own body origin, the antibodies identify them as “self” and refrain from attacking them whereas external antigens are identified as “non-self” and thus are attacked. The ability of the body to fight against these antigens is called immunity and this immunity can be either innate immunity (inborn i.e. present from birth) or acquired immunity (that we acquire in our lifetime)....

Characteristics of Antigen

There are certain properties of an antigen that makes them a good immunogen. These are:...

Structure of Antigen

The antigenic structure differs based on the size, nature, and immunogenicity of the antigen. Its molecular structure is based on its ability to bind to the antigen-binding site of the antibody. On the basis of the molecular structure called antigenic determinants or the epitope, the antibodies differentiate between different antigens. The counterpart of the epitope in antigen is the paratope in antibody which is the exact region that interacts with the antigen. The epitope and paratope combine with each other like a lock and key. The number of antibodies a single antigen can bind with depends on the number of epitopes present in that antigen. Antibodies stimulated by one antigen can even interact with different antigens and this mechanism is called cross-reactivity....

Antigen Presenting Cells

When an antigen enters a body, it needs to be identified first so that the immune system can be informed about the invasion. This identification of the antigen is done with the help of the Antigen Presenting Cells or APC. When detecting the presence of an unknown substance comes in contact with the substance engulfs it and then digests the substance in smaller fragments and then these fragments are transported to the cell’s surface with the help of MHC class I and MHC class II molecules, where these fragments will be used as an indicator for the other immune cells to inform them about the invasion. By this mechanism, these APCs inform the immune system about the antigen. Examples of APCs are B-cells before they activate and differentiate into antibodies, dendritic cells, and macrophages....

After the entry of an Antigen, what happens?

When an antigen enters the body the APCs like B-cells, macrophages, or dendritic cells come to inspect it. They engulf the antigen digests it into smaller fragments and with the help of MHC molecules present it on their surface where the T cells inspect them and then alert the whole immune system. Then the B-cells as per the epitopes of the antigen design the exact paratope containing antibodies whose job is then to find other such antigens and destroy them. On the other hand, there is another group of B-cells which after the identification of the exact type of antigen will start developing memory cells for that antigen so that in the future if another encounter occurs with the same type of antigen then antibody formation and immune response can be faster....

Types of Antigens

Antigens can be classified based on the following categories;...

Difference Between Antigen and Antibody

Antigen Antibody Any foreign particle and in some cases own body particles that can induce an immune response in the body is called an antigen. When the antigen induces an immune response in the body the immune system releases its “magic bullet” against the antigen which is called the antibody. As they act against the immune system thus are also called immunogens. They are globulin proteins of the immune system and thus are also called immunoglobulins.  They are mainly proteins and carbohydrates but lipids and nucleic acids can also act as antigens when coupled with proteins or carbohydrates.  They are proteins only.  They have specialized regions called epitopes which interact with the antibodies. interact They have specialized regions called paratopes which interact with epitopes of antigens.  They are responsible for causing disease or allergic reactions in the body.  They are responsible for protecting the body by destroying the disease or allergy-causing antigen.  Antigens can be divided into multiple classes based on their immunogenicity and origin.  Antibodies are of only five basic classes. These are Immunoglobulin G, A, M, E, and D....

FAQs on Antigen

Q1: How can you define an antigen?...