Difference Between Antigen and Antibody

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

  1. Comparison Chart
  2. Antigen
  3. Structure of an antigen
  4. Antibody
  5. Structure of an antibody

Comparison Chart

FeaturesAntigenAntibody
Denoted byAgAb
In relation to immune responseImmunogenImmunoglobulin
Composed ofCan 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
OriginationIt can be originated from outside the body as well as inside the bodyIt originates within the body
RoleIt causes diseases or allergic reactionsIt protects the immune system by lysis, phagocytosis and precipitation of an antigenic substance
ForeignnessIt is foreign molecule for a bodyIt is not
SizeVariable (large molecules are highly antigenic)Variable ,according to the different classes of immunoglobulins
Specificity/
Determinants
It’s specificity is due to ‘’ epitope’’It’s specificity is due to ‘’ paratope’’
ProcessingIt is processed by APCs ( antigen processing cells like macrophages, dendritic cells, B-cellsIt is processed by plasma B-cells
Structural diversityShows high complexityY- shaped structure
TypesEndogenous, exogenous and autoantigensIgG, IgM, IgA, IgD and IgE

Antigen

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

  • Immunogen
  • Antigenicity
  • Immunogenicity
  • Immunological reactivity
  • Antigenic determinant
  • Foreignness
  • Complete antigen
  • Haptens
  • 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.

Complete antigen:

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

Haptens:

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:

hapten types

Diagram showing the association of hapten molecule with a molecular carrier to form a complete antigen is given below:complete antigen formation
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

antigen diagramStructure 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.

Antibody

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

  • Immunoglobulin
  • 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.

Properties IgGIgAIgMIgDIgE
Constitution80% of total Ig10-15%5-10%0.2%0.1%
LocationBlood, lymph, intestineBody secretions, blood, lymphBlood, lymph,
B-cell surface
Blood, lymph,
b-cell surface
Mast cells and basophils cells
RoleProduction of complement protein, also involved in neutralization, agglutinationProvides localized protection in external secretionsInvolved in production of complement protein, agglutinationAct as b- cell receptorInvolved in hypersensitive reactions
Half life23 days6-8 days5 days3days2days
Placenta transferYesNoNoNoNo
StructureMonomerDimerPentamerMonomerMonomer
IggIgAIgMIgDIgE

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.

Plasma cells: This forms after maturation of B-cells. This considers as a hub for antibody production, which can produce antibody about 2000 antibodies per second.

Structure of an Antibody

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 H2L. H-chain and l-chain attach by “disulphide bond”.

L-chain is similar in all immunoglobulins, but H-chain is different in all immunoglobulins;

ImmunoglobulinsH-chain
IgGƳ (gamma)
IgAα (alpha)
IgMµ (mu)
IgDδ (delta)
IgEƐ (epsilon)

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.

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