What are antigens?
1915#0
In immunology, antigens (Ag) are structures (aka substances) specifically bound by antibodies (Ab) or a cell surface version of Ab ~ B cell antigen receptor (BCR). The term antigen originally described a structural molecule that binds specifically to an antibody only in the form of native antigen. It was expanded later to refer to any molecule or a linear molecular fragment after processing the native antigen that can be recognized by T-cell receptor (TCR). BCR and TCR are both highly variable antigen receptors diversified by somatic V(D)J recombination. Both T cells and B cells are cellular components of adaptive immunity. The Ag abbreviation stands for an antibody generator.
Antigen
1915#2
Also, an antigen is a molecule that binds to Ag-specific receptors, but cannot necessarily induce an immune response in the body by itself. Antigens are usually proteins, peptides (amino acid chains) and polysaccharides (chains of monosaccharides/simple sugars) but lipids and nucleic acids become antigens only when combined with proteins and polysaccharides. In general, saccharides and lipids (as opposed to peptides) qualify as antigens but not as immunogens since they cannot elicit an immune response on their own. Furthermore, for a peptide to induce an immune response (activation of T-cells by antigen-presenting cells) it must be a large enough size, since peptides too small will also not elicit an immune response.
Antigen
5951626#0
Antigenic variation refers to the mechanism by which an infectious agent such as a protozoan, bacterium or virus alters its surface proteins in order to avoid a host immune response. It is related to phase variation. Immune evasion is particularly important for organisms that target long-lived hosts, repeatedly infect a single host and are easily transmittable. Antigenic variation not only enables immune evasion by the pathogen, but also allows the microbes to cause re-infection, as their antigens are no longer recognized by the host's immune system. When an organism is exposed to a particular antigen (i.e. a protein on the surface of a bacterium) an immune response is stimulated and antibodies are generated to target that specific antigen. The immune system will then "remember" that particular antigen, and defenses aimed at that antigen become part of the immune system’s acquired immune response. If the same pathogen tries to re-infect the same host the antibodies will act rapidly to target the pathogen for destruction. However, if the pathogen can alter its surface antigens, it can evade the host's acquired immune system. This will allow the pathogen to re-infect the host while the immune system generates new antibodies to target the newly identified antigen. Antigenic variation can occur by altering a variety of surface molecules including proteins and carbohydrates. There are many molecular mechanisms behind antigenic variation, including gene conversion, site-specific DNA inversions, hypermutation, as well as recombination of sequence cassettes. In all cases, antigenic variation and phase variation result in a heterogenic phenotype of a clonal population. Individual cells either express the phase-variable protein(s) or express one of multiple antigenic forms of the protein. This form of regulation has been identified mainly, but not exclusively, for a wide variety of surface structures in pathogens and is implicated as a virulence strategy.
Antigenic variation
1915#1
Antigens are "targeted" by antibodies. Each antibody is specifically produced by the immune system to match an antigen after cells in the immune system come into "contact" with it; this allows a precise identification or matching of the antigen and the initiation of a tailored response. The antibody is said to "match" the antigen in the sense that it can bind to it due to an adaptation in a region of the antibody; because of this, many different antibodies are produced, each able to bind a different antigen while sharing the same basic structure. In most cases, an adapted antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react and bind more than one antigen.
Antigen
1704073#10
Lymphocytes are one of the five kinds of white blood cells or leukocytes, circulating in the blood. 
Although mature lymphocytes all look pretty much alike, they are extraordinarily diverse in their functions. The most abundant lymphocytes are:
B lymphocytes (often simply called B cells) and
T lymphocytes (likewise called T cells).
B cells are produced in the bone marrow.
The precursors of T cells are also produced in the bone marrow but leave the bone marrow and mature in the thymus (which accounts for their designation).
Link to drawing showing the organs of the immune system. 
Each B cell and T cell is specific for a particular antigen. What this means is that each is able to bind to a particular molecular structure.
The specificity of binding resides in a receptor for antigen:
the B cell receptor (BCR) for antigen and
the T cell receptor (TCR) respectively.
Both BCRs and TCRs share these properties:
They are integral membrane proteins.
They are present in thousands of identical copies exposed at the cell surface.
They are made before the cell ever encounters an antigen.
They are encoded by genes assembled by the recombination of segments of DNA.
How antigen receptor diversity is generated.
They have a unique binding site.
This site binds to a portion of the antigen called an antigenic determinant or epitope.
The binding, like that between an enzyme and its substrate depends on complementarity of the surface of the receptor and the surface of the epitope.
The binding occurs by non-covalent forces (again, like an enzyme binding to its substrate).
Successful binding of the antigen receptor to the epitope, if accompanied by additional signals, results in:
stimulation of the cell to leave G0 and enter the cell cycle.
Repeated mitosis leads to the development of a clone of cells bearing the same antigen receptor; that is, a clone of cells of the identical specificity.
BCRs and TCRs differ in:
their structure;
the genes that encode them;
the type of epitope to which they bind.
B Cells
Antigen processing
1915#7
At the molecular level, an antigen can be characterized by its ability to bind to an antibody's variable Fab region. Different antibodies have the potential to discriminate among specific epitopes present on the antigen surface. A hapten is a small molecule that changes the structure of an antigenic epitope. In order to induce an immune response, it needs to be attached to a large carrier molecule such as a protein (a complex of peptides). Antigens are usually carried by proteins and polysaccharides, and less frequently, lipids. This includes parts (coats, capsules, cell walls, flagella, fimbriae, and toxins) of bacteria, viruses, and other microorganisms. Lipids and nucleic acids are antigenic only when combined with proteins and polysaccharides. Non-microbial non-self antigens can include pollen, egg white and proteins from transplanted tissues and organs or on the surface of transfused blood cells.
Antigen
47052678#0
Cancer/testis (CT) antigens are a group of proteins united by their importance in development and in cancer immunotherapy. In general, expression of these proteins is restricted to male germ cells in the adult animal. However, in cancer these developmental antigens are often re-expressed and can serve as a locus of immune activation. Thus, they are often classified as tumor antigens. The expression of CT antigens in various malignancies is heterogeneous and often correlates with tumor progression. CT antigens have been described in melanoma, liver cancer, lung cancer, bladder cancer, and pediatric tumors such as neuroblastoma. Gametogenesis offers an important role for many of these antigens in the differentiation, migration, and cell division of primordial germ cells, spermatagonia spermatocytes and spermatids.
Cancer/testis antigens
22004637#0
Antigenicity is the capacity of a chemical structure (either an antigen or hapten) to bind specifically with a group of certain products that have adaptive immunity: T cell receptors or antibodies (a.k.a. B cell receptors). Antigenicity was more commonly used in the past to refer to what is now known as immunogenicity, and the two are still often used interchangeably. However, strictly speaking, immunogenicity refers to the ability of an antigen to induce an adaptive immune response. Thus an antigen might bind specifically to a T or B cell receptor, but not induce an adaptive immune response. If the antigen does induce a response, it is an 'immunogenic antigen', which is referred to as an immunogen.
Antigenicity
1661124#15
Antibodies are a key component of the adaptive immune response, playing a central role in both recognizing foreign antigens and stimulating an immune response. Antibodies are Y-shaped proteins produced by some B cells and are composed of two regions: an antigen-binding fragment (Fab), which binds to antigens, and a Fragment crystallizable (Fc) region, which interacts with so-called Fc receptors that are expressed on the surface of different immune cell types including macrophages, neutrophils and NK cells. Many immunotherapeutic regimens involve antibodies. Monoclonal antibody technology engineers and generates antibodies against specific antigens, such as those present on tumor surfaces. These antibodies that are specific to the antigens of the tumor, can then be injected into a tumor
Cancer immunotherapy
49691494#0
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