Acquired immunity does not occur unless foreign microbes or toxins enter the body. Each microbe or toxin contains one or more specific chemical substances that play an important role in triggering acquired immunity. These substances are mainly proteins, peptides, polysaccharides, or lipoproteins. These molecules or substances are called antigens.
Antigens (Ag) are substances that are foreign to the body and trigger an immune response, producing antibodies against them.
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General Properties of Antigens
Some important properties of antigens are as follows:
Chemical Nature
- Most antigens are proteins.
- Some are polysaccharides and lipoproteins in nature.
- Nucleic acids can also act as antigens if complexed with proteins and polysaccharides.
- Simple proteins (homopolymers) are usually less immunogenic.
Foreign Nature
- All antigens are foreign to the body.
- However, there are some proteins occurring within the host body that might also induce an immune response. These proteins are called autoantigens.
Molecular Size
- The molecular size of effective antigens generally greater than 10000 Da.
- The ideal antigen should have a molecular weight of approximately 100000 Da.
- Haptens are low molecular weight substances and are considered partial antigens.
Molecular Rigidity and Complexity
- Usually, rigid molecules are good antigens. They can increase the number of antibodies to certain structures.
- The structural complexity of an antigen is also plays a crucial role in its immunogenecity.
- A simple, repeating sequence of a single amino acid is less likely to trigger an immune response than a more complex sequence with multiple amino acid units.
Antigenic Determinants and Cross-reactivity
- Antigenic determinants are the specific regions of an antigen that interact with and bind to antibodies, triggering an immune response.
- Antigens usually need two or more antigenic determinants to induce antibody production. Thus, small antigens cannot induce antibodies, as they have only one antigenic determinant.
- Cros-reactivity of antigens, where antibodies induced by a different antigen can recognize and bind to another antigen.
Structure of Antigen
- The structure of antigens depends on their nature, size, and immunogenicity.
- They have a structural component known as an epitope or antigenic determinant where the antigen is involved in the reaction with antibodies.
- Most antigens consist of many epitopes.
- The region of antibodies that interacts with antigens is called a paratope or antigen binding site.
- The epitope and paratope fit together like a lock and key, showing how their specific shapes match perfectly, allowing them to bind precisely.
