Difference Between Antigen and Antibody

The difference between antigen and antibody is mainly due to the  following factors like functional role, specificity factor and foreignness.
Functional role: Antigen incites the immune system, whereas the antibody is produced in response to a specific antigen.
Specificity factor: Both antigen and antibody are specific. The specificity of antigen is due to presence of epitopes, while the specificity of an antibody is due to the existence of paratopes.

Foreignness: An antigen is a foreign particle, which induces the antibody production. As the foreignness of the antigen increases, the formation 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. Definition
  3. Structure
  4. Key Differences
  5. Conclusion

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 epitopeIt’s specificity is due to paratope
ProcessingIt is processed by APCs ( antigen presenting 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

Definition of Antigen

Antigens are the foreign substances that recognized by the 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: It can define as the any foreign substance that generates an immune response. “All immunogens can be antigens, but all antigens cannot be immunogens”.

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 can define as the specific reaction of antigen and antibody sensitised cells.

Antigenic determinant: It is also called epitope. It is another 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 principal 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, depending upon their chemical reaction.

Complete antigen: It can induce the production of an antibody alone. It can react with a specific antibody. These do not need any carrier protein molecule to induce antibody formation.

Haptens: These are also called incomplete antigens, as these cannot influence the production of the antibody alone. It cannot react specifically to an antibody, without a molecular carrier. It further categorizes 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 found 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.

Definition of Antibody

Antibodies are the large, organic, Y-shaped molecules that form in reaction to the particular antigens. Its primary function is to recognize the invaders and to destroy or neutralize them. This is the key element of an adaptive immunity.

Important terms to remember

  • Immunoglobulin
  • Humoral immunity
  • Plasma cells
  • B- lymphocytes

Immunoglobulin or Ig: It is a class of proteins that are present in the blood plasma of the immune system, and 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 direct the humoral or antibody-mediated immunity. In this kind of immunity, the antibodies formed by the plasma cells perform following activities like phagocytosis, opsonisation, neutralisation etc.

B-lymphocytes: It also refers as B-cells that originate in the bone marrow. This involves humoral immunity.

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

Structure of an Antigen

antigen diagramIt mainly consists of protrusions that are mainly called antigenic determinants (epitopes) that are very specific in nature, which only binds with a particular antibody to the antigen-binding site.

Structure of an Antibody

antibody
It consists 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 represents as H2L2. H-chain and l-chain attach by disulphide bond. L-chain is similar in all immunoglobulins, while H-chain differs 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 330 amino 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, which 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 found in the N-terminus region of an antibody, two variable region form one antigen-binding site.

Fc: It stands for fragment crystallisation, which is the remaining part of a variable region that participates in a cellular attachment.

Key Differences Between Antigen and Antibody

  1. The antigen and antibody are denoted as Ag and Ab, and also termed as immunogen and immunoglobulin.
  2. Antigens exist both inside and outside the human body, whereas antibodies form inside the human body.
  3. The major role of an antigen is to cause diseases or allergic reactions, whereas antibodies produce in response to a particular antigen that can cause lysis or precipitation of the antigenic substance.
  4. Antigens are processed by different antigen presenting cells inside our body, whereas antibodies are processed or differentiated from the B lymphocytes.
  5. The antigens possess high complexity in their structure, while the antibodies exists as Y-shaped structure.

Conclusion

Therefore, we can conclude that the study of the antigens and antibodies helps us to determine the potency of the foreign body, and to know the kind of immunity developed in our body in reaction to that specific antigen.

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