Each antibody binds to a specific antigen; an interaction similar to a lock and key

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Each antibody binds to a specific antigen; an interaction similar to a lock and key.

An antigen is a substance or molecule that when introduced into the body result the production of an antibody by the immune system which will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. This substance can be part of a living organism (bacterium or parasite) but can also be a virus or a chemical toxin.. Originally the term came from antibody generator[1][2] and was a molecule that binds specifically to an antibody, but the term now also refers to any molecule or molecular fragment that can be bound by a major histocompatibility complex (MHC) and presented to a T-cell receptor[3].

"Self" antigens are usually tolerated by the immune system; whereas "Non-self" antigens are identified as intruders and attacked by the immune system.

Autoimmune disorders: arise from the immune system reacting to its own antigens.

Similarly, an immunogen is a specific type of antigen. An immunogen is defined as a substance that is able to provoke an adaptive immune response if injected on its own[4]. Said another way, an immunogen is able to induce an immune response, while an antigen is able to combine with the products of an immune response once they are made.

Immunogenicity is the ability to induce a humoral and/or cell-mediated immune response

Antigenicity is the ability to combine specifically with the final products of the [immune response] (i.e. secreted antibodies and/or surface receptors on T-cells). Although all molecules that have the property of immunogenicity also have the property of antigenicity, the reverse is not true."[5]

At the molecular level, an antigen is characterized by its ability to be "bound" at the antigen-binding site of an antibody. Note also that antibodies tend to discriminate between the specific molecular structures presented on the surface of the antigen.

Antigens are usually proteins or polysaccharides. This includes parts (coats, capsules, cell walls, flagella, fimbrae, and toxins) of bacteria, viruses, and other microorganisms. Lipids and nucleic acids are antigenic only when combined with proteins and polysaccharides. Non-microbial exogenous (non-self) antigens can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells. Vaccines are examples of immunogenic antigens intentionally administered to induce acquired immunity in the recipient.

Epitope –ADS antigen determinant site the small site or molecular surface features of an antigen capable of being bound by an antibody (antigenic determinant).

They determine the antigen specificity, any change in the ADS are associated with change in the antigen specificity.

The antigen may have identical or different( chemical structure, configuration, number and distribution )ADS. In the later form the antigen is more immunogenic.

Antigenic molecules, normally being "large" biological polymers, usually present several surface features that can act as points of interaction for specific antibodies. Any such distinct molecular feature constitutes an epitope. Most antigens therefore have the potential to be bound by several distinct antibodies, each of which is specific to a particular epitope. Using the "lock and key" Different antibody idiotypes, each having distinctly formed complementarity determining regions, correspond to the various "locks" that can match "the keys" (epitopes) presented on the antigen molecule.

Antigen specificity:

It is the function of simple chemical groups on the surface of antigen molecule(ADS ).

It is the antigen ability to react with the specific antibody based on chemical structure, configuration, number and distribution of epitopes.

Allergen - A substance capable of causing an allergic reaction. The allergic reaction may result after exposure via ingestion, inhalation, injection, or contact with skin.

Superantigen - A class of antigens which cause non-specific activation of T-cells resulting in polyclonal ( large fraction )of T cell activation and massive cytokine release. The diseases associated with exposure to superantigen are, in part, due to hyperactivation of the immune system and subsequent release of biologically active cytokines by activated T-cell

Examples of super antigens include:

Staphylococcal entertoxins( food poisoning)

Staphylococcal TSS.

Streptococcal pyrogenic exotoxins (shock).

Tolerogen - A substance that invokes a specific immune non-responsiveness due to its molecular form. If its molecular form is changed, a tolerogen can become an immunogen.

Hapten: incomplete antigen

Due to small molecular weight, which are too small to stimulate the immune response.

It is become immunogenic when coupled with carrier. A carrier macromolecule is an immunogenic molecule, usually protein,

Antigen valency

It is the number of epitopes on the antigen surface.

Heterophil (heterogenic ) antigens :

These are group of identical Ags sharing similar ADS and found on unrelated M.O, animals and plants.

e.g M protein of streptococcus and that of the protein of the heart.

Forssman Ags ( these antigen are identical Ags found on RBcs of many animal species

Antigen shift and drift

Some microbial agents (e.g. influenza and AIDS ) can change its surface antigens slightly (antigen drift ) or radically (shift ). Through the Ag shift new strains can introduced

This process renders the management and control by vaccination program unsuccessful.

T-independent antigens

These are antigens, which can directly stimulate the B- cells to produce antibody without requirement of T- cell help,

Polysaccharides are T –independent Ag.

The response to these Ag differs from the response to other Ag.

These Ag are characterized by the same ADS repeated many times (polymeric structure).

These Ag are generally resistant to degradation and thus, they persist for long periods of time and continue to stimulate the immune system.

T- dependent antigens:

T-dependent antigens are those that do not directly stimulate the production of antibody without the help of T-cells.

-Proteins are T- dependent antigens.

-structurally these antigens are characterized by few copies of many different ADS.

Exogenous antigens

Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation, ingestion, or injection. The immune system's response to exogenous antigens by endocytosis or phagocytosis, exogenous 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. B-lymphocytes either direct or indirect ( T helper ) which carry CD4

Endogenous antigens

Endogenous antigens are antigens that have been generated within previously normal cells 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 MHC class I molecules. If activated cytotoxic CD8+ T cells recognize them, the T cells begin to secrete various toxins that cause the lysis or apoptosis of the infected cell.



1- Major histocompatibility Ag MHC

They are inherited Ag & it can stimulate immune response in genetically different individial



_ present on APC

_ present on all nucleated cell in body

_ Carry exogenous Ag

_Carry endogenous Ag

_ To T-helper which carry CD4 or directly to B lymphocyte

_ To T cytotoxic cell which carry CD8

2- Blood group Ag

There are glucoprotein in nature found on the surface of RBCs

Ab in serum against

Carry (Ag)



A- antigen



B- antigen



(universal receptor)

A&B- antigen



(universal donor)




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.

Tumor antigens

Tumor antigens or neoantigens are those antigens that are presented by MHC I or MHC II molecules on the surface of tumor cells. These antigens can sometimes be presented by tumor cells and never by the normal onesresult from a tumor-specific mutation


Antigens are defined as substances recognized by the body as foreign, causing the body to produce an antibody to react specifically with it

Characteristics of antigens: 

In order to be an antigen to you it must be foreign (not found in the host): THE MORE FOREIGN THE BETTER ANTIGEN!

  • Autologous antigens are  your own antigens (not foreign to you)

  • Homologous, or allogenic, antigens are antigens from someone else (within the same species) that are foreign to you

Antigens must be chemically complex. 

  • Proteins and polysaccharides are antigenic due to their complexity.  On the other hand, lipids are antigenic only if coupled to protein or sugar. 

  • Besides being chemically complex, antigens must also be large enough to stimulate antibody production.  Their molecular weight needs to be at least 10,000 dalton.

  • Due to the complexity of these molecules there are specific antigenic determinant sites, or epitopes, which are those portions of the antigen that reacts specifically with the antibody.

Factors determining whether an antigen will stimulate an antibody response:

  • Degree of foreignness.  Only human blood is transfused to humans.  

  • Size and complexity.  Although red cells are smaller than white blood cells, they tend to be more antigenic due to the complexity of the antigens on the cell surface.  Some are proteins and others are oligosaccharides.

  • Dose of antigen administered. How much antigen is the individual exposed to and what is the frequency of that exposure.

  • Genetic makeup of host may also dictate whether an antibody is produced.  Some individuals have a greater ability to make antibody and others have the antigen so they would not make the antibody.

  • .



The adaptive immunity response

1-Humeral immune response (directed against the extracellular) produced antibody and memory cell. It is function of B-cell.

2-Cell mediated response (directed against the intracellular) produced cytotoxity. It is function of T-cell.

The adaptive immunity also can classified into two types based on how it is acquired by the host into active and passive immunity.

Comparison between active and passive immunity

Passive immunity

Active immunity

By injection of antibody

Naturally: by maternal immunity through the transfer of antibody from mother to her son(placenta or colistrum)

Artifically: by hyperimmune serum

Passive role

Begins immediately

Relatively short

It is dangerous may cause allergy (serum sickness )

By injection of immunogen

Naturally: by infection

Artifically: by vaccine

Active role

Begins after a lag phase of 7-15 days.


Produce stronger and rapid protection


-Role of the host immune system




Effect of the 2nd dose

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