The substance used to produce immunity against disease in the living body:

There are several substances involved in the process of producing immunity against diseases in the living body. However, the main substance used for this purpose is a vaccine. Let's understand the different terms and their roles in detail:

i) Antigen:
- Antigens are substances that can stimulate the immune system of the body to produce an immune response.
- They are usually foreign substances such as bacteria, viruses, or other microorganisms that invade the body.
- Antigens are recognized by the immune system as "non-self" and trigger the production of specific antibodies to fight against them.

ii) Immune:
- The term "immune" refers to the immune system, which is a complex network of cells, tissues, and organs that work together to defend the body against infections and diseases.
- The immune system is responsible for recognizing and destroying harmful substances, including antigens, to maintain overall health and well-being.

iii) Vaccine:
- A vaccine is a biological preparation that contains weakened or killed forms of antigens.
- It is designed to stimulate the immune system to recognize and remember specific antigens, without causing the disease itself.
- When a vaccine is administered, the immune system recognizes the antigens present in the vaccine as foreign and mounts an immune response.
- This immune response leads to the production of antibodies and the development of memory cells, which provide long-term immunity.
- Vaccines help prevent the occurrence of diseases by preparing the immune system to respond quickly and effectively when it encounters the actual pathogen in the future.

iv) Antibody:
- Antibodies, also known as immunoglobulins, are proteins produced by the immune system in response to the presence of antigens.
- Antibodies specifically bind to antigens, neutralize them, and mark them for destruction by other immune cells.
- They play a crucial role in the immune response by recognizing and targeting specific antigens, thereby protecting the body from infections and diseases.

Summary:
In summary, while antigens are foreign substances that trigger an immune response, the immune system is responsible for defending the body against diseases. Vaccines contain antigens that stimulate the immune system and help develop immunity. Antibodies, on the other hand, are produced by the immune system in response to antigens and play a key role in neutralizing and eliminating pathogens.