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Antigen (Ag)

What Does Antigen (Ag) Mean?

An antigen is any foreign matter that can trigger an immune response in the body. It is a broad category that includes bacteria, viruses, parasites, chemicals, and toxins.

Safeopedia Explains Antigen (Ag)

Antigens cause bodily reactions by activating the white blood cells or leukocytes responsible for fighting diseases and infections.

What Are Antigens Made Of?

They are composed of proteins, peptides, and polysaccharides and contain distinct sites on their surface called epitopes.

The host's immune system generates antibodies to combat antigen. The antibodies interact with specific epitopes via antigen binding sites called paratopes.

Types of Antigens

Antigens are divided into two main groups:

Foreign Antigens

As the name suggests, these antigens come from outside the body. They are also known as heteroantigens and are present in bacteria, viruses, parasites, certain food proteins, cells from other people, and toxins.


These antigens are present within the body and do not trigger an immune response in healthy individuals. In some cases, however, the body mistakenly acts against them, causing autoimmune inflammation.

The Immune System's Response to Antigens

The epitope-paratope interaction between antigens and antibodies dictates the specific characteristic of the immune response. The adaptive immune response can be of the following types.


Immunity is triggered when antibodies expressed on the B cell surface recognize specific epitopes of an antigen and internalize the antigen. The B cell surface presents the antigen where it is recognized by helper T cells that subsequently activate the B cell which rapidly divides to produce plasma cells and memory B cells


The antigen-presenting cells internalize and digest the antigen and present the fragments on their cell surface through a major histocompatibility complex or MHC. T cells on recognizing the MHC-antigen complex start secreting cytokines that facilitate the maturation of T cells. The T cells mature into helper T cells to produce more cytokines or cytotoxic T cells that attack and destroy pathogen-infected cells.


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