What is an antigen?

An antigen is defined as any substance which can stimulate our body to make an immune response against that substance.

Example Bacteria, Viruses, Chemicals


  1. Antigens are foreign in nature.
  2. Antigens are chemical (Proteins, polysaccharides)
  3. Antigens have a minimum molecular mass of 5000 Da.
  4. The complex structure of an antigen defines its immunogenicity (capability of inducing an immune response)
  5. Antigens are Species, Organ-specific.

Antigen structure 

The structure of an antigen is characterized by the ability of antigens to bind to the antigen-binding site of an antibody. An antigen can be differentiated by the molecular mass. In most cases the antigens are proteins. It can include capsules, flagella, or other microorganisms. Antigens can be lipids (usually lipids are non-immunogenic, but they can be functions as antigen and known as Haptens.)

Types of Antigens:

  1. Based on origin
    1. Exogenous antigens are the antigens that are eternally originated from the host and are foreign to the host. There are several methods by which exogenous antigens can enter a host like inhalation, ingestion, or injection
    2. Endogenous antigens are the antigens that are the body’s cells or antigenic products. These are produced during metabolism or viral or bacterial infection. These cells are processed in the macrophages and then detected by Cytotoxic T-cells. These antigens might result in autoimmune diseases.
    3. Auto-antigens are the self-protein of the host cell that are attacked by the host’s immune system and it leads to autoimmune disease. These antigens can be deadly for the host because the body’s cells are targets by the immune system.
    4. Tumor Antigens (neoantigens) are present on the surface of tumor cells that induce an immune response in hosts. These antigens are presented by MHC I and II and are results of tumor-specific mutation during malignant tumor.
  2. Based on immune response:
    1. Immunogen/Complete antigens: Antigens are defined by stimulation of an immune response, though all antigens might not capable of inducing an immune response. Antigens that can stimulate an immune response are known as
    2. Haptens/Incomplete antigens: derived from the Greek word “Haptein” which means “To fasten”. Haptens itself can’t function as an immunogen. But when they are bound to a carrel molecule they can work as a complete antigen.

Antigens vs immunogen



Antigens are substances that specifically bind to antibody or B and T cells An immunogen is also an antigen and capable of inducing an immune response
An antigen can be immunogenic or non-immunogenic Immunogenic nature
All antigens are not immunogen All immunogen are antigens
An antigen can be protein, polysaccharides, lipids, or nucleic acids. Immunogens are protein and polysaccharides


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