Bone Marrow: Red blood cells, many types of white blood cells, and platelets are produced from stem cells found in the spongy interior of the bones. Every day, the bone marrow produces billions of new blood cells and releases them into the blood. B- cells and T- cells are also produced in bone marrow. The B- cell maturation takes place in the bone marrow.
Thymus: This tiny organ, located in the upper chest below the breast bone, is responsible for T- cell maturation.
Lymph Nodes and Vessels: The lymphatic system in our body contains lymph vessels and lymph nodes. The networks of lymph vessels are connected by lymph nodes. These tiny nodes filter and kill pathogens to prevent germs from spreading to other body areas. Lymph nodes have collections of B cells and T cells throughout the body. Cells collect in lymph nodes to communicate with each other. Lymph nodes can become swollen when they are fighting an infection.
Spleen: Spleen is a lymphoid organ located near stomach. Spleen filter blood, capture pathogens in blood, and fight blood infections. The spleen has a collection of B cells, T cells, and monocytes.
Liver: The liver is the major organ responsible for producing proteins of the complement system. In addition, it contains large numbers of phagocytic cells that eat bacteria in the blood as it passes through the liver.
Tonsils and adenoids: The adenoids and tonsils function as a part of the body's immune system. Acting as our first line of defence, they trap and kill harmful pathogens like bacteria and viruses that enter our mouth and nose. These are made up of lymphoid tissue which is densely filled with lymphocytes.
Skin: The human skin serves as a barrier against microbial invasion, protecting the body from potential pathogens. The skin's primary defensive features include: The outermost layer of the skin is called epidermis. It is composed of tightly packed cells that provide a physical barrier. Skin cells contain a tough protein Keratin, making them resistant to abrasion and water loss. The skin surface has a slightly acidic pH (around 5.5), which inhibits the growth of many pathogens. The process of shedding dead skin cells helps remove attached microbes. Dermis of skin contains oil glands, sweat glands, hair follicles, receptors, nerves and blood vessels. Oil and sweat play important role in inhibiting and killing microorganisms.
Mucous membrane: Mucous membranes line digestive and respiratory tracts. The digestive tract mucous membrane kills bacteria present in food by producing stomach acid and digestive enzymes. The respiratory system mucous membrane can trap and remove airborne microbes. It has ciliated epithelium and produces mucus. Mucus traps dust, microbes, and other particles and cilia remove them from respiratory system.

9.2 ORGANS OF THE IMMUNE SYSTEM