15. Antigens. Types of antigens. Antigenic characteristics of microorganisms. Humoral immunity. Characteristics of antibodies (immunoglobulins). Structure and function of different immunoglobulin classes. Mechanism of action of antibodies. Local immunity. Monoclonal antibodies.

Antigens: Structure and Properties

• Antigens (Ag) are substances capable of stimulating an immune response, often leading to antibody production.
• The specific part of the antigen recognised by an antibody or T-cell receptor is called the epitope or antigenic determinant.
• Antibodies typically recognise conformational epitopes (complex, three-dimensional structures), while T cells recognise linear peptides when presented by MHC molecules.
• A Hapten is a small molecule that is not immunogenic on its own but can trigger an immune response when attached to a larger carrier protein.
• Properties (Antigenicity): Antigenicity refers to the ability of a substance to bind with an antibody or immune cell receptor. A stronger antigenicity means it can better elicit an immune response.
• Foreignness: Antigens must generally be foreign to the body to trigger an immune response. This means they must come from outside the host (e.g., bacteria, viruses, or foreign proteins).
• Size: Larger molecules, such as proteins, are typically stronger antigens compared to smaller molecules like haptens.
• Complexity: Antigens with complex structures, like proteins with multiple amino acids or sugars, tend to elicit stronger immune responses.
• Solubility: Soluble antigens often trigger a better immune response than insoluble ones.
• Stability: Stable antigens are more likely to elicit a consistent immune response.