Immune System in Humans - Practice Questions & MCQ

Immune Response:

• The immune response involves primary immune response and secondary immune response.

Primary Immune Response:

• 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.

• 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.