Innate immunity and the organs of the immune system

How do we stay well?

Every day you meet germs on your skin, in the air you breathe and in the food you eat. Yet you are not ill every day. Why not?

The power of the body to fight the organisms that cause disease is immunity. There are two kinds. Innate immunity is present from birth. Acquired immunity is gained during life, and the next lesson explains it.

Innate immunity

Innate immunity is a non-specific defence. It works in the same way against any germ, whether it has met that germ before or not. It has four kinds of barriers.

• Physical barriers. The skin is a wall as long as it is not cut. The mucus that coats the lining of the respiratory, digestive and urogenital tracts traps the microbes that try to enter.
• Physiological barriers. The acid of the stomach, the saliva and the tears prevent the growth of microbes. Tears and saliva also hold an enzyme, lysozyme, which breaks the wall of bacteria.
• Cellular barriers. The white cells that act as a barrier are the polymorphonuclear leucocytes (PMNL, mainly the neutrophils), the monocytes, the natural killer cells (a type of lymphocyte) and the macrophages. A macrophage is a monocyte that has left the blood and entered a tissue. These cells destroy the germs by phagocytosis, which means that they swallow and digest them.
• Cytokine barriers. A cell that is infected by a virus makes interferons. They are cytokines, which are chemical signals between cells. The interferons protect the cells that are not yet infected from the virus.

You should find that three barriers, each stopping most of the invaders, leave very few. Every barrier works on what the one before it let pass.

The immune system: organs and cells

The immune system is the set of organs, tissues and cells that defend the body. Its cells are the lymphocytes, the main cells of acquired immunity, together with the phagocytes. The organs that make and house the lymphocytes are the lymphoid organs.

Primary lymphoid organs. Here the immature lymphocytes develop into mature cells that can recognise an antigen. An antigen is a substance that the body can respond to, usually a part of a pathogen.

• The bone marrow is the main lymphoid organ. All the blood cells, including the lymphocytes, are made there. The B cells mature in the bone marrow.
• The thymus is a lobed organ near the heart, under the breastbone. It is large at birth and shrinks with age. The T cells mature in the thymus.

Secondary lymphoid organs. Here the mature lymphocytes meet the antigens and are activated.

• The spleen is a large, bean shaped organ. It holds lymphocytes and phagocytes. It acts as a filter of the blood, trapping microbes that are carried in it. It also holds a large store of red blood cells.
• The lymph nodes are small, solid structures along the lymphatic system. They trap microbes and antigens that reach the lymph and tissue fluid. The trapped antigens activate the lymphocytes there.
• The tonsils, the Peyer's patches of the small intestine and the appendix are other secondary organs.
• The mucosa associated lymphoid tissue (MALT) lies in the lining of the respiratory, digestive and urogenital tracts. It makes up about half of all the lymphoid tissue of the body. These tracts are the main doors by which germs enter, so it is a good place for a guard.

Try it in Python

The program passes the invaders through three barriers in turn.

Practise with new numbers

Common mistakes

• Saying that innate immunity is specific. It is the same against every germ.
• Putting the skin among the cellular barriers. The skin and the mucus are physical barriers.
• Putting the acid of the stomach among the physical barriers. It is a physiological barrier.
• Saying that interferons are antibodies. They are cytokines, made by cells that are infected by a virus.
• Mixing up the primary and the secondary lymphoid organs. The bone marrow and the thymus are primary. The spleen and the lymph nodes are secondary.

Remember these

• Innate immunity: present at birth, non-specific. Four barriers: physical, physiological, cellular, cytokine.
• Cells: neutrophils (PMNL), monocytes, macrophages and natural killer cells. They destroy germs by phagocytosis.
• Interferons: made by virus infected cells, they protect the cells not yet infected.
• Primary lymphoid organs: bone marrow and thymus. Secondary: spleen, lymph nodes, tonsils, Peyer's patches, appendix, MALT.
• MALT is about half of the lymphoid tissue of the body.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four kinds of barriers of innate immunity, with one example of each.

2. What are interferons, and who makes them?

3. Which are the primary lymphoid organs, and why are they called primary?

4. What is the work of the spleen?

5. What is MALT, and what share of the lymphoid tissue does it make?

References

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 21, The Lymphatic and Immune System.
• Parker N. et al. (2016). Microbiology, OpenStax, the chapter on innate nonspecific host defences.
• NCERT (2023). Biology, Textbook for Class XII, chapter 7.