After initial contact with an antigen, no antibodies are present for a period of several days.
Then, a slow rise in the antibody titer occurs.
This is called the primary immune response.

Secondary Immune Response:

Memory cells may remain in the body for decades.
Every new encounter with the same antigen results in a rapid proliferation of memory cells.
This is also called “booster response”.
This accelerated, more intense response is called the secondary immune response.

Lines of Defence:

There are three lines of defence in the body:
- First line of defence: It includes physical barriers and physiological barriers of innate immunity.
- Second line of defence: It includes cellular barriers and cytokine barriers of innate immunity
- Third line of defence: It includes acquired immunity.

Immune System in Humans

The human immune system consists of lymphoid organs, tissues, cells and soluble molecules like antibodies.
The immune system is unique in the sense that it recognizes foreign antigens, responds to these and remembers them.
The immune system also plays an important role in allergic reactions, auto-immune diseases and organ transplantation.

Lymphoid organs:

These are the organs where origin and/or maturation and proliferation of lymphocytes occur.
There are two types of lymphoid organs in the human body:
- Primary lymphoid organs
- Secondary lymphoid organs

Primary Lymphoid Organs:

The primary lymphoid organs are bone marrow and thymus where immature lymphocytes differentiate into antigen-sensitive lymphocytes.
The bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced.
The thymus is a lobed organ located near the heart and beneath the breastbone.
The thymus is quite large at the time of birth but keeps reducing in size with age and by the time puberty is attained it reduces to a very small size.
Both bone-marrow and thymus provide micro-environments for the development and maturation of T-lymphocytes.

Secondary Lymphoid Organs:

After maturation the lymphocytes migrate to secondary lymphoid organs like spleen, lymph nodes, tonsils, Peyer’s patches of small intestine and appendix.
The secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells.
The spleen is a large bean-shaped organ. It mainly contains lymphocytes and phagocytes.
It acts as a filter of the blood by trapping blood-borne microorganisms. Spleen also has a large reservoir of erythrocytes.
The lymph nodes are small solid structures located at different points along with the lymphatic system.
Lymph nodes serve to trap the microorganisms or other antigens, which happen to get into the lymph and tissue fluid.
Antigens trapped in the lymph nodes are responsible for the activation of lymphocytes present there and cause the immune response.
There is lymphoid tissue also located within the lining of the major tracts (respiratory, digestive and urogenital tracts) called mucosa-associated lymphoid tissue (MALT).
It constitutes about 50 percent of the lymphoid tissue in the human body.