Read this article to learn about the definition, types and structure of antigens in our body!
Definition:
Antigens are substances which, when introduced into the body, stimulate the production of antibodies.
Chemical Nature:
The antigens are mostly the conjugated proteins like lipoproteins, glycoproteins and nucleoproteins.
Structure:
Antigenic determinants or epitopes (Gk. epi – upon, topos- place) are components of antigen. Each antigen carries many epitopes. Each Y-shaped antibody molecule has atleast two binding sites that can attach to a specific epitope on an antigen. An antibody can also bind to identical epitopes of two different cells at the same time which can cause neighbouring cells to aggregate. Antigens combine with the antibody. The combination is very much like the lock and key analogy.
Types:
Based upon the ability of antigens to carry out their functions, antigens are of two types: complete antigens and incomplete antigens (haptens). A complete antigen is able to induce antibody formation and produce a specific and observable reaction with the antibody so produced.
Haptens (Gr. hapten to grasp; partial antigens) are substances which are incapable of inducing antibody formation by themselves, but can be capable of inducing antibodies on combining with larger molecules (normally proteins) which serve as carriers.
Antigens which are present on the body’s own cells are called the auto-antigens or self antigens. The antigens on the non-self cells are known as foreign antigens or non-self antigens.
H antigen:
Red blood corpuscles of all ABO blood groups possess a common antigen, the H antigen, which is a precursor for the formation of A and В antigens. Due to universal distribution, H antigen is not ordinarily important in grouping or blood transfusion.
However, Bhende et al (1952) from Mumbai reported a very rare example in which A and В antigens and H antigens were absent from the red blood corpuscles. This is known as Bombay or Oh blood group. Such individuals will have anti A, anti В and anti H antibodies. Therefore, they can accept the blood only from their own group.
Antigen Presenting Cells (APCs):
The cells that can engulf antigen and present fragments to T cells are called antigen presenting cells (APCs).
There are three types of antigen presenting cells in the body: macrophages, dendritic cells and В cells.
1. Macrophages:
Macrophages are usually found in a resting state. Their phagocytic capabilities are greatly increased when they are stimulated to become activated macrophages. The macrophages are present alongwith lymphocytes in almost all the lymphoid tissues, e.g., monocytes as blood macrophages and histocytes as tissue macrophages.
2. Dendritic Cells:
These cells are characterized by long cytoplasmic processes. Their primary role is to function as highly effective antigen-trapping and antigen presenting cells. These cells are nonphagocytic in nature. They are found in lymph nodes, spleen, thymus and skin. The different types of dendritic cells are:
(i) Langerhan’s dendritic cells in epidermis of skin which trap the organisms coming in contact with body surface.
(ii) Dendritic cells in spleen, which trap the antigen in blood.
(iii) Follicular dendritic cells in lymph nodes which trap the antigen in the lymph.
Thus macrophages and dendritic cells play an important role in the trapping and presentation of antigens to T and В cells to initiate the immune response.
Steinman was awarded Nobel Prize (2011) for his discovery of the dendritic cell and its role in adaptive immunity.
3. B-cells:
В-cells express on their surface intra-membrane immunoglobulin (Ig) molecules that function as В cell antigen receptors. Since all the receptors on a single В cell are identical, each В cell can bind only one antigen. This makes them much more efficient antigen-presenting cells than macrophages, which must ingest any foreign material that comes their way.
Descendants of В-cells (plasma cells) produce antibodies.