Antigens
An antigen is any substance capable of stimulating an immune response and binding specifically to an antibody.
Examples:
- Viruses (HIV, HBV, HCV)
- Bacteria
- Parasites
- Hormones
- Tumor markers
- Drugs
Types of Antigens
- Exogenous Antigens
- Enter from outside the body.
- Example: Hepatitis B Surface Antigen (HBsAg)
- Endogenous Antigens
- Produced within cells.
- Example: Viral proteins are synthesized in infected cells.
- Autoantigens
- Normal body proteins that trigger autoimmune reactions.
Antibodies (Immunoglobulins)
Antibodies are proteins produced by B lymphocytes in response to antigen exposure.
Structure of an Antibody
An antibody consists of:
- Two heavy chains
- Two light chains
- Variable region (antigen binding site)
- Constant region
Functions of Antibodies
- Neutralize pathogens
- Activate complement
- Promote phagocytosis
- Prevent the spread of infection
Classes of Immunoglobulins
IgG
- Most abundant antibody.
- Indicates past infection or immunity.
- Crosses the placenta.
IgM
- The first antibody is produced after infection.
- Indicates recent or acute infection.
IgA
- Found in saliva, tears, breast milk, and mucosal surfaces.
IgE
- Associated with allergies and parasitic infections.
IgD
- Functions mainly as a receptor on B-cells.