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Antigens are molecules that evoke an immunological response from the body. Most antigens are proteins but they can be almost any molecule including carbohydrates, DNA and RNA. Because many proteins are unique to bacterial and virus species, antigens act like identification tags for unwanted or dangerous invaders and are the means by which the immune system identifies what it should destroy.

Auto-antigens are antigens in the body's own tissue. Why the immune system targets its own tissue is unclear but it seems to be the driving force behind autoimmune diseases such as multiple sclerosis.

The acquired immune system is that part of the immune system which learns to recognise and respond to specific viruses, bacteria and fungi - collectively known as pathogens. The acquired immune system responds to pathogens by binding to small sections of their broken down proteins. Proteins are made up of small building blocks called amino acids which are directly coded for in the DNA. The amino acids join together like beads on the string of a necklace to form the proteins. Sections of proteins are known as peptides or peptide strings. Those peptides which elicit an immune response are known as antigens. The small sections of antigens that the cells of the acquired immune system binds to are called epitopes.

The acquired immune system can be divided into two parts, cell-mediated and humoral immunity. Both parts of the acquired immune system work through types of white blood cells (leukocytes) called lymphocytes. Their are two kinds of lymphocytes - T-cells and B-cells.

Cell mediated immunity works through T-cells, each of which have thousands of identical receptors on its surface known as T-cell receptors (TCR). Each TCR binds to a sub-section of an antigen called an epitope or antigenic determinant. The T-cells are presented with antigens through a doughnut-shaped molecule called the major histocompatibility complex (MHC).

MHC comes in two varieties MHC class I and MHC class II. Almost all cells in the human bodies have MHC class I molecules on their surfaces which present antigens through their "doughnut holes" to a type of T-cell called a killer T-cell. Some specific cells, for example macrophages, B-cells and dendritic cells have MHC class II molecules on their surfaces. Such cells are called antigen presenting cells (APC). They present antigens through their holes to another type of T-cell called a helper T-cell.

Humoral immunity works via antibodies which bind to the epitopes on antigens either directly or indirectly through an intermediate molecule called a hapten. When an antibody binds to an antigen, it may either render it inactive just by being bound to it, or act as a signal to yet other types of immune system cells, called phagocytes, to engulf (phagocytose) it.

Multiple Sclerosis is thought to be primarily a cell-mediated autoimmune disease operating via helper T-cells which target antigens in the proteins that make up myelin.

Antigen links:
HON Allergy Glossary Antigen
Antigen Receptors
Antigen Presentation
Antibody Recognition of Antigen
immunology - MHC complex

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