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ANTIGENS

Antigens are foreign matter in the body administered through blood . Antigenic proteins also exist  in different  forms and  even in the same species , these are called  allotypes .  

The human body can produce millions of different antibodies  recognizing  different types of  foreign antigens . It does these by shuffling  different sections of parts of the genes that produce the variable region. All cells posses antigen molecules  on the cell surface membrane . These molecules acts as surface markers (molecules that are unique  to that particular cells ), these surface  markers(antigens)  makes the  cells to recognize  each other .

The body can distinguish  its own (self ) from foreign antigens (non self).Normally  the body makes  antibodies  against non self  antigens  except in autoimmunity  where the body starts  making antibodies against  its own antigens  

 Microorganisms  like  bacteria  carry  antigens  on their  surfaces  which the human  immune  cells  recognize and acts appropriately.

Antibodies are specific to particular antigens, though cross reaction between different antibodies and  different antigens  that are usually not specific  for each other is also possible ., these is  probably due to their structural similarities  e.g. antibodies of horse serum reacts with donkeys serum protein.  In fact the more related the two species are , the greater the chances  of their serological similarities .

Similarly even proteins from different parts of the same species have different functions (e.g. hemoglobin’s differ serologically from proteins in the kidney ) these shows that most organs  posses special proteins or carbohydrates  that are only peculiar  to themselves .

 Reactivates of antigens resides in  a small prosthetic group  called haptens . Haptens are substances that are incapable  of inducing antibody formation  when alone but  when they are attached to ordinary immune proteins  and polysaccharides , they induce antibodies against themselves

Characteristics of antigens include:

1. Immunogenicity: Antigens have the ability to induce an immune response by stimulating the production of antibodies or activating specific immune cells, such as T cells or B cells.

2. Antigenicity: Antigens can specifically bind to immune receptors, known as antigen receptors, present on the surface of B cells and T cells. This binding is essential for the recognition and initiation of an immune response.

Types of Antigens:

1. Foreign Antigens: These are antigens derived from sources external to the body, such as pathogens or environmental substances. Foreign antigens are recognized by the immune system as non-self and can trigger an immune response.

2. Self-Antigens: Self-antigens are molecules derived from the body’s own cells and tissues. They are normally tolerated by the immune system, as the immune system has mechanisms to distinguish self from non-self. However, in some cases, the immune system may recognize self-antigens as foreign, leading to autoimmune reactions and autoimmune diseases.

3. Autoantigens: Autoantigens are self-antigens that trigger an immune response in autoimmune diseases. In these conditions, the immune system mistakenly recognizes and attacks the body’s own tissues.

4. Heterophile Antigens: Heterophile antigens are antigens that are structurally similar among different species. They can induce cross-reactivity, leading to immune responses against related antigens in different organisms. Examples include the heterophile antigens used in serological tests for infectious diseases like mononucleosis.

5. Neoantigens: Neoantigens are antigens that arise from genetic mutations or modifications within the body. They can be found in cancer cells or cells infected with certain viruses. Neoantigens are recognized by the immune system as abnormal, leading to immune responses against these cells.