An antigen is a substance that provokes an immune response when introduced to the body, causing antibodies to be produced that specifically react with that antigen. Antigens can be complete antigens, which can induce antibody formation on their own, or haptens, which must be linked to a carrier protein to become immunogenic. Haptens are divided into complex haptens, which form precipitates with antibodies, and simple haptens, which do not. Antigenicity depends on factors like the antigen's size, chemical nature, and ability to be broken down by tissue enzymes. Superantigens can interact non-specifically with antigen-presenting cells and T-lymphocytes to stimulate immune responses.

1. ANTIGEN
DEFINITION:-
 An antigen is a substance which , when introduced into a body evokes immune response to
produce a specific antibody with which it reacts in an observable manner.
I. TYPES OF ANTIGEN
 They may be classified as
A. Complete Antigen
B. Haptens/Incomplete Antigen:
 Complex haptens
 Simple haptens
A. Complete Antigen
These are substances which can induce antibody formation by themselves and can react specifically with
these antibodies.
B. Haptens
 Haptens are substances unable to induce antibody formation on its own but can become
immunogenic (capable of inducing antibodies) when covalently linked to proteins, called carrier
proteins.
 However,haptens can react specifically with induced antibodies.
 These antibodies are produced not only against the hapten but also against the carrier protein.
 Complex haptens can combine with specific antibodies to form precipitates e.g. capsular polysaccharide
of pneumococc1.
 2. Simple haptens combine with specific antibodies but no precipitate is produced. This is due to
univalent character of simple haptens, whereas complex haptens are polyvalent.
 Proantigens are low molecular weight substances which do not induce antibody formation but can
cause delayed hyper-sensitivity reaction e.g. dinitrochlorobenzene (DNCB).
 Antigenic determinant (epitope) is the smallest unit of antigenicity.
 The combining site on the antibody molecule, corresponding to the epitope, is called the paratope

2. FACTORS OF ANTIGENICITY
1. Foreignness
2. Size
3. Chemical nature
4. Susceptibility to tissue enzymes
5. Antigenic specificity
6. Species specificity
7. Isospecificity
8 . Autospecificity
9. Organ specificity
10 . Heterophile specificity.
 Weil-Felix reaction and Paul-Bunnell test are based on the presence of heterophile antibodies.
 Forssman Antigen
 It is a lipoprotein-polysaccharide complex widely present in man, animals, birds and bacteria.
SUPERANTIG
 Superantigens are molecules that can interact with antigen-presenting cells (APCs) and T-lymphocytes
in a non-specific manner.
 These antigens do not involve the endocytic processing as required in typical antigen presentation.
 Staphylococcal enterotoxins, exfoliative toxin and some viral proteins are examples of superantigens.
THANK YOU
DHANANJAY SINGH