HUMORAL IMMUNITY

 

- B Lymphocytes participate in humoral immunity.

- on exposure to antigens  à  activation of B- lymphocytes to plasma cells à produce  

   specific antibodies against foreign antigens.

Mechanism pf Humoral immunity

  - Antigen is fragmented by macrophages.

  - A fragment of antigen binds to MHC Class II protein in the cell membrane of macrophages and forms a complex.

  - this complex is presented to the B cell by macrophages ( Antigen Presenting cell).

  - Macrophage also secretes interleukin 1 which activates both T-Helper cells and B-

     Lymphocytes.

  - ‘T’ helper cells secrete IL₂, IL₄, IL₅, IL₆ (cytokines).

  - these cytokines act on ‘B’ lymphocytes.

  - ‘B’ Lymphocytes undergo proliferation & differentiation

  - ‘B’ lymphocytes form plasma cells.

  - Plasma cells produce specific antibodies.

Antibodies: Types Ig G, Ig A, Ig M, Ig D, Ig E.

1. Ig G antibodies:

            - 75% of the total antibodies in the body.

            - can cross the placenta.

            - secreted into colostrum

            - role in immunity in the fetus & the new – born.

2. Ig A Antibodies:

            - 20% of total antibodies.

            - present in mucosal secretions.

            - protect the mucosal surface from infections.

3. Ig M antibodies:

            - Mostly intra vascular.

            - Destroy the organisms that enter the circulation.

4. Ig D antibodies

            - Present in the surface of immature B- lymphocytes.

            - help in the functional maturation of B- Lymphocytes.

5. Ig E antibodies:

                        - Present mainly on the surface of mast cells and basophils.

                        -  Responsible for hypersensitive immune reactions (Allergy)

Functions of Antibodies:

 

1.      Neutralisation of Antigen : - Antibodies neutralise the toxic effect of some bacterial

toxins.

2.      Immobilization of bacteria:  Immobilise the cilia or flagella of motile bacteria.  This limits the spread of disease.

3.      Enhancement of phagocytosis: (Opsonisation):

                    - Ig G antibodies form an attachment to the antigen and enhances the phagocytic

                      activity of neutrophils and macrophages.

              4.   Antibody dependant cellular cytotoxicity:

                   - Antibodies link the target cells with Natural killer cells which kill the targets     

                       secreting toxic chemicals.

               5.  Agglutination & Precipitation of antigen:

                    By cross linking antibodies make the pathogens to clump together (agglutination) &

                     the soluble antigens to form precipitation.  This helps in easy phagocytosis.          

                6.  Activation of complement:

       - Antibodies bound to antigens activate a group of proteins in the plasma called  

        complement proteins.  They facilitate the exudation of phagocytes towards the site

        of infection.

                    7. Provide fetal & newborn immunity:

                     - By the transfer of antibodies from mother through placenta & colostrums, the

                         fetus & newborn acquire immunity.