Immunity:

The capacity to recognize the intrusion of any material foreign to the body and to mobilize cells and cell products to help remove the particular sort of foreign material with greater speed and effectiveness is called immunity.

Immune System:

In animals in addition to physical barriers (skin + mucous membranes) and phagocytes, there is a third mechanism, to defend their bodies against the foreign invaders; this is called immune system.

Components of Immune system:

The components of immune system include the lymphocytes (B and T) and the antibodies – which are special type of proteins. These antibodies are immunoglobulins which are synthesized by vertebrates, in response to antigens; and immobilize it, or sets in motion events that ultimately cause its destruction.

Lymphocyte T and B have been named due to their relationship with Thymus gland, and Bursa of Fabricius respectively. The influence of the thymus gland is essential in making T-cells immunologically competent. Bursa of Fabricius is lymphoid structure present in the wall of cloaca of young birds from where B-lymphocytes were discovered to have role in immune system. 
 

Antigen or Immunogen:

Antigen or Immunogen is a foreign substance, often a protein which stimulates the formation of antibodies. Antibodies are specific i.e. cause the destruction of the antigen, which stimulated their production. Antibodies are manufactured in B-lymphocytes, then secreted into the lymph and blood where they circulate freely.

Structure of antibody:

(a)

 (a) An antibody molecule consists of four polypeptide chains – two identical light chain and two identical heavy chain – linked by disulfide (- S – S -) bridges. Variable amino acid sequences (V) in the light chains and upper regions of the heavy chains determine which antigen will bind to that particular antibody. Constant amino acid sequences (C) are the same for all the antibodies in one class.

(b)

(b) Large antigens-antibody complexes will form if there are multiple copies of the antigenic molecule on the foreign cell’s surface.  

         

        Cell Mediated Response

          

       Humoral Immune Response

 

T-cells recognize antigens, then combat micro-organisms and / or effect the rejection of foreign tissues (in case of tissue transplant). This is called cell-mediated response.

B-cells recognize antigen and form plasma cell clone. These plasma cells synthesize and liberate antibodies into the blood plasma and tissue fluid. Here antibodies attach to the surface of bacteria and speed up their phagocytosis, or combine with and neutralize toxins produce by micro-organisms, by producing antitoxins. This is called humoral immune response.

 

Vaccination:

When we get vaccination against a specific disease (antigen), we become immune to that infection or disease. If we get vaccination against, polio, smallpox, measles, mumps etc., once in our home time, we are protected or become immune to that infection in our future life.


Types of immunity:

Active immunity:

The use of vaccines, which stimulate the production of antibodies in the bodies, and making a person immune against the disease or infection, is called active immunity. There are two types of active immunity:

  

Artificially induced active           immunity

 

      

  Naturally induced immunity

But this active immunity has been achieved by artificially introducing, antigens in the body , so it is called artificially induced active immunity.

 But, when a person is exposed to an infection (antigen) – becomes ill, and in most cases survives then this immunity, developed against that disease is called natural induced immunity or auto immune response.

 

Passive immunity:

In contrast to achieve immunity, in which case antigens are introduced to stimulate the production of antibodies, by artificial or natural method; antibodies are injected in the form of antisera, to make a person immune against a disease, this is called passive immunity.

In body, antigen – antibody are complexes are formed which are taken up by phagocytes and destroyed. The patient is spared the complications (or possibly death) caused by the infection or venom.

Passive immunity response is immediate, but not long lasting. Because no time is taken for the production of sufficient level of antibodies, (as antibodies are being injected) and after the level antibodies is reduced or they are used up – no more antibodies production is there.

The method of passive immunization is used to combat active infection of, tetanus, infection hepatitis, rabies, snake bite venom etc. In this case of snake bite venom passive immunity is produced by the antitoxins – so the serum is called Antivenom serum. Antiserum is a serum containing antibodies.