Types of Antigen

Antigens are classified based on their origin, structure, and the immune response they produce. The various types of antigens are explained in detail below:

Based on Origin

Antigens based on their origin are classified into three main types: exogenous, endogenous, and autoantigens.

Exogenous Antigens

• Definition: Exogenous antigens are those that originate from outside the body.
• Sources: These include bacteria, viruses, fungi, pollen, dust, and environmental toxins.
• Mechanism: Exogenous antigens are taken up by antigen-presenting cells (APCs) through processes like phagocytosis or endocytosis. They are then processed and presented on the cell surface with Major Histocompatibility Complex (MHC) class II molecules to T-helper cells, initiating an immune response.

Also Read: Difference Between T Cells and B Cells

Endogenous Antigens

• Definition: Endogenous antigens are those that are generated within the body.
• Sources: These antigens can be normal cellular proteins, proteins produced by cells infected with viruses or intracellular bacteria, or tumor antigens resulting from mutations in normal cells.
• Mechanism: Endogenous antigens are processed and presented by the infected or abnormal cells themselves. The antigens are displayed on the cell surface bound to MHC class I molecules. This complex is recognized by cytotoxic T-cells (CD8+ T cells), which then target and destroy the infected or abnormal cells to prevent the spread of infection or cancerous growth.

Autoantigens

• Definition: Autoantigens are normal body components that are mistakenly recognized as foreign by the immune system.
• Sources: These are usually normal proteins or complexes of proteins that are naturally present in the body.
• Mechanism: Under normal circumstances, the immune system is tolerant to self-antigens, but in autoimmune diseases, this tolerance breaks down, leading to an immune response against the body’s own tissues.

Based on Structure

Antigens can be classified based on their structure into four main types: proteins, polysaccharides, lipids, and nucleic acids.

Proteins

• Definition: Protein antigens are large, complex molecules made up of long chains of amino acids folded into specific three-dimensional shapes.
• Characteristics
  • Highly immunogenic due to their complex structure.
  • Can present multiple epitopes (the part of the antigen recognized by the immune system).
  • Capable of producing strong and specific immune responses.
• Examples
  • Viral proteins like those from influenza or HIV.
  • Bacterial toxins such as diphtheria toxin.
  • Enzymes and structural proteins.

Polysaccharides

• Definition: Polysaccharide antigens are composed of long chains of sugar molecules linked together.
• Characteristics
  • Less immunogenic compared to proteins but can still trigger significant immune responses.
  • Often found on the surfaces of bacteria, where they form part of the bacterial capsule.
  • Induce T-independent immune responses, meaning they can result in antibody production without the help of T-cells.
• Examples
  • Capsular polysaccharides of bacteria like Streptococcus pneumoniae and Neisseria meningitidis.
  • Cell wall components of fungi and some bacteria.

Lipids

• Definition: Lipid antigens are fat-like molecules that can become immunogenic when associated with proteins or polysaccharides.
• Characteristics
  • By themselves, lipids are usually poor immunogens.
  • When conjugated with proteins or polysaccharides, they can stimulate a more robust immune response.
  • Recognized by a specialized subset of T-cells known as CD1-restricted T-cells.
• Examples
  • Glycolipids in the cell walls of Mycobacterium tuberculosis.
  • Lipopolysaccharides (LPS) in the outer membrane of Gram-negative bacteria, which are potent endotoxins.

Also Read: Difference Between Endotoxins and Exotoxins

Nucleic Acids

• Definition: Nucleic acid antigens are composed of DNA or RNA molecules.
• Characteristics
  • Generally less immunogenic on their own.
  • Can become immunogenic when complexed with proteins or during certain infections.
  • Often involved in autoimmune responses where the immune system targets the body’s own nucleic acids.
• Examples
  • Viral genomes.
  • Autoantigens in autoimmune diseases like systemic lupus erythematosus (SLE), where anti-DNA antibodies are produced.

Based on the Immune Response

Antigens can be classified based on the type of immune response they produce: T-dependent antigens and T-independent antigens. Here’s a detailed explanation of each type:

T-dependent Antigens

• Definition: T-dependent antigens require the assistance of T-helper cells (CD4+ T-cells) to produce an antibody response from B-cells.
• Characteristics
  • These are protein antigens.
  • They are processed and presented by antigen-presenting cells (APCs) on MHC class II molecules.
  • The interaction between APCs and T-helper cells leads to T-cell activation and the production of cytokines.
  • These cytokines then help activate B-cells to produce specific antibodies against the antigen.
  • This type of immune response results in the production of high-affinity antibodies and memory B-cells, leading to long-lasting immunity.
• Examples
  • Proteins from viruses like the influenza virus hemagglutinin.
  • Bacterial toxins such as tetanus toxin.

T-independent Antigens

• Definition: T-independent antigens can stimulate B-cells to produce antibodies without the help of T-helper cells.
• Characteristics
  • These are polysaccharides or lipid antigens.
  • They have repeating epitopes that can cross-link B-cell receptors (BCRs) directly, leading to B-cell activation.
  • This immune response is generally faster but less robust compared to T-dependent responses.
  • It predominantly produces IgM antibodies and does not generate memory B-cells, resulting in shorter-lived immunity.
• Examples
  • Polysaccharides from bacterial capsules like those of Streptococcus pneumoniae and Haemophilus influenzae type b.
  • Lipopolysaccharides (LPS) from the outer membrane of Gram-negative bacteria.

Based on the Genetic Makeup

Antigens can also be classified based on genetic makeup into alloantigens, xenoantigens, and autoantigens.

Alloantigens

• Definition: Alloantigens are antigens that are present in some but not all individuals of the same species. They are genetically determined and can vary between individuals.
• Example
  • Blood group antigens
  • Histocompatibility antigens
• Significance: Important in blood transfusions, organ transplantation, and maternal-fetal interactions.

Xenoantigens

• Definition: Xenoantigens are antigens that are found in different species. These antigens are recognized as foreign by the immune system of another species.
• Example
  • Animal tissues or organs: For example, pig heart valves used in human surgery.
  • Proteins from other species are used in biotechnology or medical treatments.
• Significance: In xenotransplantation, where organs or tissues from animals are transplanted into humans, the xenoantigens can cause strong immune responses leading to rejection.

Autoantigens

• Definition: Autoantigens are normal proteins or complexes of proteins that are recognized as foreign by the immune system of the same individual, leading to an autoimmune response.
• Sources: Self-proteins
• Significance: In autoimmune diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis, and type 1 diabetes, the immune system mistakenly attacks the body’s own tissues.

Special Types

Special types of antigens include superantigens and neoantigens.

Superantigens

• Definition: Superantigens are a class of antigens that result in excessive activation of the immune system.
• Example
  • Produced by certain bacteria, viruses, and other pathogens.
  • Common examples include bacterial toxins such as Staphylococcal enterotoxins and Streptococcal pyrogenic exotoxins.
• Mechanism: Bind directly to MHC molecules and T-cell receptors, bypassing the normal antigen processing, leading to massive cytokine release and severe immune reactions.
• Significance
  • They can cause severe immune responses, including toxic shock syndrome, food poisoning, and autoimmune diseases.
  • The cytokine storm can lead to symptoms like fever, rash, and shock, and can be life-threatening.

Neoantigens

• Definition: Neoantigens are novel antigens that arise from mutations in normal cellular proteins.
• Sources
  • These antigens are often found in cancer cells due to genetic mutations that alter normal proteins.
  • They can also arise from viral infections where viral proteins are integrated into the host genome.
• Mechanism
  • Neoantigens are processed and presented by MHC molecules on the surface of cancer cells or virus-infected cells.
  • Because they are different from the body’s normal proteins, they can be recognized as foreign by the immune system, especially by T-cells.
• Significance
  • Play an important role in cancer immunotherapy, as they provide specific targets for immune-based treatments like cancer vaccines, checkpoint inhibitors, and adoptive T-cell therapies.
  • The unique nature of neoantigens makes them excellent targets for personalized medicine, where therapies are made to the individual’s specific tumor profile.

Antigens are substances that the immune system recognizes as foreign and can trigger an immune response. They can be proteins, peptides, polysaccharides, or other macromolecules. Types of antigens include self-antigens, which are recognized by the immune system as belonging to the body, and non-self antigens, such as those from pathogens or foreign substances.

Antigens are important for the immune system’s ability to distinguish between self and non-self, initiating immune responses to protect the body from harmful invaders. In this article, we will study the antigen, its types, and its functions in detail.