Types of antigen on the basis of source and immune response


Antigens are molecules that can be recognized by the immune system and trigger an immune response. The term antigen derives from the words "antibody generator". Antigens can be proteins, polysaccharides, lipids, nucleic acids, or any other substances that can bind to specific receptors on immune cells. Antigens can be classified into different types based on their source, origin, or the type of immune response they elicit.

The immune system is composed of two main branches: the innate immunity and the adaptive immunity. The innate immunity is the first line of defense against pathogens and foreign substances. It consists of physical barriers (such as skin and mucous membranes), chemical mediators (such as complement and interferons), and cellular components (such as macrophages, neutrophils, and natural killer cells). The innate immunity is fast, nonspecific, and does not have memory.

The adaptive immunity is the second line of defense that is activated by the recognition of specific antigens. It consists of two types of lymphocytes: B cells and T cells. B cells produce antibodies that bind to antigens and neutralize them or mark them for destruction. T cells can either help B cells to produce antibodies (helper T cells) or directly kill infected cells or cancer cells (cytotoxic T cells). The adaptive immunity is slow, specific, and has memory.

The recognition of antigens by the adaptive immunity involves two types of molecules: the major histocompatibility complex (MHC) and the antigen receptors. The MHC molecules are proteins that are expressed on the surface of most nucleated cells. They present fragments of antigens to T cells. There are two classes of MHC molecules: MHC class I and MHC class II. MHC class I molecules present endogenous antigens (antigens that originate from within the cell) to cytotoxic T cells. MHC class II molecules present exogenous antigens (antigens that originate from outside the cell) to helper T cells.

The antigen receptors are proteins that are expressed on the surface of B cells and T cells. They bind to specific antigens with high affinity and specificity. There are two types of antigen receptors: the B cell receptor (BCR) and the T cell receptor (TCR). The BCR is composed of two identical heavy chains and two identical light chains that form a Y-shaped structure. The variable regions at the tips of the Y-shaped structure form the antigen-binding site. The BCR can bind to free antigens in solution or on the surface of pathogens. The TCR is composed of one alpha chain and one beta chain that form a heterodimer. The variable regions at the ends of the alpha and beta chains form the antigen-binding site. The TCR can only bind to antigens that are presented by MHC molecules.

The interaction between antigens and antigen receptors initiates a cascade of signaling events that leads to the activation, proliferation, differentiation, and effector functions of B cells and T cells. The activated B cells and T cells can also form memory cells that can respond more quickly and effectively to subsequent encounters with the same antigen.

In this article, we will discuss the different types of antigens based on their source/origin and their immune response.