Explain the human immune system.

(N/A) The human immune system consists of lymphoid organs, tissues, cells, and soluble molecules like antibodies. Our immune system is unique in that it recognizes foreign antigens, responds to them, and remembers them. The immune system also plays an important role in allergic reactions, autoimmune diseases, and organ transplantation.
Lymphoid Organs: These are the organs where origin and/or maturation and proliferation of lymphocytes occur. The primary lymphoid organs are the bone marrow and the thymus, where immature lymphocytes differentiate into antigen-sensitive lymphocytes.
After maturation, the lymphocytes migrate to secondary lymphoid organs like the spleen, lymph nodes, tonsils, Peyer's patches of the small intestine, and the appendix. The secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells.
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.
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. The spleen also has a large reservoir of erythrocytes.
Lymph nodes are small solid structures located at different points along the lymphatic system. Lymph nodes 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 $50$% of the lymphoid tissue in the human body.