what do u mean by antigens?(give a simple definition)?

Answer 1

An antigen is any substance that causes your immune system to produce antibodies against it. An antigen may be a foreign substance from the environment such as chemicals, bacteria, viruses, or pollen. An antigen may also be formed within the body, as with bacterial toxins or tissue cells.
Please see the web pages for more details on Antigen.

Answer 2

Antigens Definition

Answer 3

An antigen is any substance foreign to the body and when it enters the body, it stimulates the production of a very specific protein called antibody that can react with the inducing antigen.
Examples of antigen:
pollen grains (in case of allergies), typhoid vaccine, blood group antigens A and B, etc.

Answer 4

Antigens are substances that are capable of causing the production of antibodies for an infection in the body. Antigens may or may not lead to an allergic reaction.

Answer 5

An antigen is a molecule that stimulates an immune response. The word originated from the notion that they can stimulate antibody generation. We now know that the immune system does not only consist of antibodies. The modern definition encompasses all substances that can be recognized by the adaptive immune system. This includes viruses as well as bacteria.

* Tolerogen - An antigen that invokes a specific immune non-responsiveness due to its molecular form. If its molecular form is changed, a tolerogen can become an immunogen.
* Allergen - An allergen is a substance that causes the allergic reaction. The (detrimental) reaction may result after exposure via ingestion, inhalation, injection or contact with skin.

Cells present their antigens to the immune system via a histocompatibility molecule. Depending on the antigen presented and the type of the histocompatibility molecule, several types of immune cells can become activated.

Exogenous antigens

Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation, ingestion, or injection. By endocytosis or phagocytosis, these antigens are taken into the antigen-presenting cells (APCs) and processed into fragments. APCs then present the fragments to T helper cells (CD4+) by the use of class II histocompatibility molecules on their surface. Some T cells are specific for the peptide:MHC complex. They become activated and start to secrete cytokines. Cytokines are substances that can activate cytotoxic T lymphocytes (CTL), antibody-secreting B cells, macrophages and other particles.

[edit] Endogenous antigens

Endogenous antigens are antigens that have been generated within the cell, as a result of normal cell metabolism, or because of viral or intracellular bacterial infection. The fragments are then presented on the cell surface in the complex with class I histocompatibility molecules. If activated cytotoxic CD8+ T cells recognize them, the T cells begin to secrete different toxins that cause the lysis or apoptosis of the infected cell. In order to keep the cytotoxic cells from killing cells just for presenting self-proteins, self-reactive T cells are deleted from the repertoire as a result of central tolerance (also known as negative selection which occurs in the thymus). Only those CTL that do not react to self-peptides that are presented in the thymus in the context of MHC class I molecules are allowed to enter the bloodstream.

There is an exception to the exogenous/endogenous antigen paradigm, called cross-presentation.

[edit] Autoantigens

An autoantigen is usually a normal protein or complex of proteins (and sometimes DNA or RNA) that is recognized by the immune system of patients suffering from a specific autoimmune disease. These antigens should under normal conditions not be the target of the immune system, but due to mainly genetic and environmental factors the normal immunological tolerance for such an antigen has been lost in these patients.

[edit] Tumor antigens

Tumor antigens are those antigens that are presented by the MHC I molecules on the surface of tumor cells. These antigens can sometimes be presented only by tumor cells and never by the normal ones. In this case, they are called tumor-specific antigens (TSAs) and typically result from a tumor specific mutation. More common are antigens that are presented by tumor cells and normal cells, and they are called tumor-associated antigens (TAAs). Cytotoxic T lymphocytes that recognized these antigens may be able to destroy the tumor cells before they proliferate or metastasize.

Tumor antigens can also be on the surface of the tumor in the form of, for example, a mutated receptor, in which case they will be recognized by B cells.

Answer 6

An antigen is a molecule that stimulates an immune response. The word originated from the notion that they can stimulate antibody generation.

Answer 7

Any substance that stimulates the immune sustem. Usually a protein or peptide, can sometimes be a polysaccharide (a complex sugar) or lipopolysaccharide (a complex sugar with fats attached or associated) or even a metal - some people are sensitive to nickel in particular. May stimulate both antibody production or the cellular system, or both.

Answer 8

I think its a personal thing. Life seems to be haphazard and unpredictable and we humans are endowed with a peculiar sense of acceptance when it comes to questions such as this. Your can get an accurate reading from Q & A as to how each individual responds to life. Some hate it. Some love it. Some don't care. Most live it. The degree to which we can live it with an open mind and a sense of wonder at its variability usually determines how we ultimately feel about it. Good wording on this question and thanks for asking it.

Answer 9

Antigens are substences recognized by the immune system and cause the immune system to create antibodies.

Answer 10

Any substance capable of eliciting an immune response and stimulates the production of antibodies.