The key difference: The Antigen and Antibody are the protein structures which shows a significant difference between the following factors like functional role, specificity factor and foreignness.
Antigen incites the immune system whereas the antibody produced in response to a specific antigen.
Both antigen and antibody are specific in nature but the specificity of antigen is due to epitopes whereas the specificity of an antibody is due to paratopes.
The antigen is the foreign particle which induces the antibody production. as the foreignness of the antigen increases, the production of the antibodies will be more.
In simple words, we can say that an antigen attacks the immune system and an antibody protects the immune system.
Content: Antigen Vs Antibody
|In relation to immune response||Immunogen||Immunoglobulin|
|Composed of||Can be composed of protein, lipid, carbohydrate and nucleic acid. But majority of them are proteins & polysaccharides which are most ‘’antigenic’’||Composed of conjugated proteins like
Glycoprotein, lipoprotein etc
|Origination||It can be originated from outside the body as well as inside the body||It originates within the body|
|Role||It causes diseases or allergic reactions||It protects the immune system by lysis, phagocytosis and precipitation of an antigenic substance|
|Foreignness||It is foreign molecule for a body||It is not|
|Size||Variable (large molecules are highly antigenic)||Variable ,according to the different classes of immunoglobulins|
|It’s specificity is due to ‘’ epitope’’||It’s specificity is due to ‘’ paratope’’|
|Processing||It is processed by APCs ( antigen processing cells like macrophages, dendritic cells, B-cells||It is processed by plasma B-cells|
|Structural diversity||Shows high complexity||Y- shaped structure|
|Types||Endogenous, exogenous and autoantigens||IgG, IgM, IgA, IgD and IgE|
Definition: These are the foreign substances that recognize by particular antibodies. All the antigens cannot induce an immune response but those which are capable, are known as immunogens. The antigen can be bacteria, virus, fungi etc., other than that it can be dust, pollen grains.
Important terms to remember
- Immunological reactivity
- Antigenic determinant
- Complete antigen
- T-cell dependent antigen
- T- cell-independent antigen
Immunogen: Any foreign substance that generates an immune response. “All immunogens can be an antigen, but all antigens cannot be immunogen”.
Antigenecity: It is the property of an antigen that has the ability to bind with specific antibody or cell surface receptors.
Immunogenecity: It is the property of an antigen to generate an immune response.
Immunological reactivity: It is defined as the specific reaction of antigen and antibody sensitised cells.
Antigenic determinant: Also called “epitope”. It is also a property of an antigen that is specific to a specific antibody. It consists of a 4-5 amino acid or monosaccharide residues. An arrangement of an epitope can be sequential or linear.
These are specific in nature, and specificity is due to:
- Specific chemical nature
- Specific electrical charge
Foreignness: This is the major property of an antigen which determines the antigenicity of a molecule, i.e. Higher is the degree of foreignness higher is its antigenicity.
Types of Antigen
There are two types of antigen based on their chemical reaction namely complete and hapten antigens.
It can induce the production of an antibody alone. And, It can react with a specific antibody. These do not need any carrier protein molecule to induce antibody formation
Also called “incomplete antigen’” as it cannot influence the production of the antibody alone. It cannot react specifically to an antibody, without a molecular carrier. It is further categorises into two types:
Diagram showing the association of hapten molecule with a molecular carrier to form a complete antigen is given below:
T-cell dependent antigen: These are the protein antigens which can activate b-cells only by the help of t-cells. It can produce all immunoglobulins; IgG, IgM, IgA, IgD and IgE. Its structure is quite complex, and this kind of antigenicity founds in most of the pathogenic microorganisms like a virus.
T-cell independent antigen: Other than protein molecule, this contains a polysaccharide that can directly activate the b-cells without the help of t-cells. It can produce only IgM antibody. Its structure is simple.
Structure of an Antigen
Structure of an antigen mainly consists of protrusions that are mainly called antigenic determinant (epitopes) that are very specific in nature, which only binds with a particular antibody to the antigen binding site.
Definition: These are a large, organic, Y-shaped molecule that forms in reaction to particular antigens. Its main function is to recognise the invaders and to destroy or neutralise them. This is the key element of adaptive immunity.
Important terms to remember
- Humoral immunity
- Plasma cells
- B- lymphocytes
Immunoglobulin: It denotes as “Ig”, which are a class of proteins that are present in the blood plasma of the immune system, functions as an antibody. “All antibody can be immunoglobulins, but all immunoglobulins cannot be antibody”.
There are five classes of immunoglobulin; – IgG, IgM, IgA, IgD and IgE.
|Constitution||80% of total Ig||10-15%||5-10%||0.2%||0.1%|
|Location||Blood, lymph, intestine||Body secretions, blood, lymph||Blood, lymph, |
|Mast cells and basophils cells|
|Role||Production of complement protein, also involved in neutralization, agglutination||Provides localized protection in external secretions||Involved in production of complement protein, agglutination||Act as b- cell receptor||Involved in hypersensitive reactions|
|Half life||23 days||6-8 days||5 days||3days||2days|
Humoral immunity: Antibodies that direct “antibody-mediated immunity” is called humoral immunity. In this kind of immunity where antibodies releases by plasma cells perform following activities like phagocytosis, opsonisation, neutralisation etc.
B-lymphocytes: Also called “B-cells” that are formed in the bone marrow. This involves humoral immunity.
Structure of an Antibody
Structure of antibody consist of 4 polypeptide chains:
- Two heavy chains(H-chain):- 440 amino acid long
- Two light chains (I- chain):- 220 amino acid long
Hence, an antibody is represented as H2L2 . H-chain and l-chain attach by “disulphide bond”.
L-chain is similar in all immunoglobulins, but H-chain is different in all immunoglobulins;
Cr: It stands for “constant region”. It is the lower part of the Y-shaped antibody that is invariable in nature. For ƴ, α and δ:-Cr is 330amino acid long and for µ & ɛ: – Cr is 440 amino acid long.
Vr: It stands for “variable region”. It is present on the terminal ends of both h-chain and l- chain of an antibody that is variable in nature. In each class of immunoglobulin, variable region differs, called as “hypervariable region” (5-10 amino acid long).
Fab: It stands for “fragment antigen binding-site” that founds in the N-terminus region of an antibody, two variable region form one antigen binding site.
Fc: It stands for “fragment crystallisation”, that is the remaining part of a variable region involves in a cellular attachment.