Parts of the immune system

Your immune system defends the body from infection. It is made up of a complex network of cells, tissues and organs in your body. An underactive or overactive immune system can cause health issues.

The main parts of the immune system are:

• White blood cells
• Antibodies
• Complement system
• Lymphatic system
• Spleen
• Bone marrow
• Thymus.

White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders.

Antibodies are specialized, Y-shaped proteins that bind like a lock-and-key to the body’s foreign invaders — whether they are viruses, bacteria, fungi or parasites.

Complement system is made up of a large number of distinct plasma proteins that react with one another to opsonize pathogens and induce a series of inflammatory responses that help to fight infection. A number of complement proteins are proteases that are themselves activated by proteolytic cleavage.

Lymphatic system is a collection of organs involved in the production, maturation, and harboring of white blood cells called lymphocytes. It also includes a network of vessels that transport or filter the fluid known as lymph in which lymphocytes circulate. The figure below shows major lymphatic vessels and other structures that make up the lymphatic system. Besides the tonsils, organs of the lymphatic system include the thymus, the spleen, and hundreds of lymph nodes that are distributed along the lymphatic vessels.

The Spleen main function is to act as a filter for the blood. It recognizes and removes old, malformed, or damaged red blood cells. When blood flows into the spleen, the spleen performs “quality control”; the red blood cells must pass through a maze of narrow passages. Healthy blood cells simply pass through the spleen and continue to circulate throughout the bloodstream. Blood cells that can’t pass the test will be broken down in the spleen by macrophages. Macrophages are large white blood cells that specialize in destroying these unhealthy red blood cells.

Bone marrow is the spongy tissue inside some of your bones, such as your hip and thigh bones. It contains stem cells. The stem cells can develop into the red blood cells that carry oxygen through your body, the white blood cells that fight infections, and the platelets that help with blood clotting.

The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, Thymus cell lymphocytes or T cells mature. … The thymus is located in the upper front part of the chest, in the anterior superior mediastinum, behind the sternum, and in front of the heart.

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The body’s other defenses against microbes

As well as the immune system, the body has several other ways to defend itself against microbes, including: 

• Skin – a waterproof barrier that secretes oil with bacteria-killing properties
• Lungs – mucous in the lungs (phlegm) traps foreign particles, and small hairs (cilia) wave the mucous upwards so it can be coughed out
• Digestive tract – the mucous lining contains antibodies, and the acid in the stomach can kill most microbes
• Other defenses – body fluids like skin oil, saliva and tears contain anti-bacterial enzymes that help reduce the risk of infection. The constant flushing of the urinary tract and the bowel also helps.