The human immune system works as an armour against infections and diseases. It helps the body distinguish between non-self cells like bacterial, fungal and viral pathogens from self-cells. The immune system responds to non-self agents and remembers them. It also plays an important role in reacting to allergens organ transplantation as well as auto-immune responses. The human immune system consists of lymphoid organs, lymph nodes and lymphocytes. The lymphoid organs are where lymphocytes originate, mature and proliferate. Lymphoid organs can be classified as primary lymphoid organs and secondary lymphoid organs. Bone marrow and the thymus are two primary lymphoid organs. Bone marrow is a flexible tissue found in the hollow interior of bones where all blood cells including lymphocytes are produced. The thymus, on the other hand, is a lobed organ, which is located near heart and beneath the breastbone. The thymus, which is quite large at birth, shrinks in size with an increase in age. By the time a person reaches puberty, the thymus shrinks considerably. Both bone marrow and the thymus provide the environment for the development and maturation of T-lymphocytes. When lymphocytes mature, they move to the secondary lymphoid organs. It is inside the secondary lymphoid organs that lymphocytes interact with antigens and proliferate to form effector cells. The spleen is a large bean-shaped organ located in the upper-left part of the abdomen, protected by the rib cage. It chiefly contains lymphocytes, erythrocytes and phagocytes . It acts as a large reservoir of blood to be supplied in time of emergencies like hemorrhagic shock. It also plays the role of RBC graveyard by removing old or damaged RBCs from the body. Apart from the lymphoid organs, the human body has clusters of lymph nodes in places such as the neck, underneath the arms and in the groin. Lymph nodes trap microbes and antigens that enter lymph and tissue fluid, which activate the lymphocytes to produce antibodies, resulting in an immune response. Another lymphoid tissue called MALT, is located inside the epithelial lining of major tracts like the respiratory, digestive and urogenital tracts. The T-cells, B-cells and macrophages present in MALT help regulate mucosal immunity. Therefore, the immune system, which collectively comprises the lymphoid organs, lymph nodes and tissues, protects the human body from invasion by pathogens and microbes.