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Lymphoid organs review

Overview of primary (thymus and bone marrow) and secondary (spleen, tonsils, Peyer's patch, appendix, MALT) lymphoid organs, and their functions.
This article offers an overview of:
  • Primary lymphoid organs (bone marrow and thymus) and
  • Secondary lymphoid organs (including the spleen, lymph nodes, and MALT)

Key terms

Lymphoid organAn organ where lymphocytes are produced and develop
LymphocytesWhite blood cells, or immune cells, like B and T lymphocytes
ThymusPrimary lymphoid organ where T lymphocytes mature
Bone marrowPrimary lymphoid organ where immune cells are produced and B lymphocytes mature
SpleenSecondary lymphoid organ that filters blood and stores erythrocytes and lymphocytes
Lymph nodesSecondary lymphoid organ that filters lymph
MALTMucosa-associated lymphoid tissue, a type of secondary lymphoid organ

Parts of the human immune system

We've learnt about the types of immunity that help our system defend against foreign objects (immune system review). But what is the immune system itself made of? Can you think of a body part that protects you from pathogens?
The parts of the immune system range from physical barriers and substances (innate defenses like skin, saliva etc), to cells and organ systems. Lymphoid organs play a very important role in the development of immune cells, and also act as checkpoints of foreign substances.
Let's take a better look at these organs.

Primary lymphoid organs

Primary lymphoid organs are where B and T lymphocytes form, multiply, and mature.

Bone marrow

Bone marrow is the spongy tissue found inside bones where most immune cells are produced. While B lymphocytes mature within the marrow, immature T lymphocytes migrate to the thymus for further development.


The thymus is a bilobed structure below the breastbone, near the heart, where T lymphocytes mature and multiply. In most humans, the thymus increases in size until puberty, after which it shrinks and is eventually replaced by fatty tissue.
Let's do a quick review.
Test your understanding 1
Bone marrow transplants are carried out in patients suffering from chronic illnesses, immune deficiencies or undergoing cancer treatment.
Which of the following events will be improved in a patient who has received such a transplant?
Choose 3 answers:

Secondary lymphoid organs

Secondary lymphoid organs filter extracellular fluids for antigens and microorganisms. They are the sites at which lymphocytes are activated by antigens, and subsequently develop into effector cells.

Lymph nodes

Lymph nodes are small, solid tissues found along lymphatic vessels (the small, green dots in the above image). Lymph fluid is filtered for antigens, which are trapped and then exposed to lymphocytes in the nodes.


The spleen is a bean-shaped organ that stores erythrocytes, lymphocytes and phagocytes. It filters the bloodstream by trapping blood-borne pathogens in its cells.


Mucosa-associated lymphoid tissue (MALT) is found close to the epithelial linings of major systems, including the respiratory, digestive and urogenital tracts. MALT makes up nearly half of the lymphoid tissue in the body, and allows lymphocytes to interact with antigens.
Other secondary lymphoid organs include the tonsils, Peyer’s patches of small intestine and the appendix.
Let's do a few more reviews.
Test your understanding 2
Accidents and some diseases can cause the spleen to rupture, often leading to its removal.
Which of the following functions will be compromised by the removal of the spleen?
Choose 2 answers:

Test your understanding 3
The human immune system is illustrated in the image below.
Which of the labelled structures is responsible for trapping pathogens present in lymph?
Choose 1 answer:

Common misconceptions

  • All lymphoid organs filter lymph. The term 'lymphoid' can be confusing; it actually refers to any place in which lymphocytes are found. Only tissues along lymphatic vessels (like lymph nodes) filter lymph fluid.
  • Primary lymphoid organs carry out primary immune responses. Lymphoid organs are named for the stages of lymphocyte development and function, and not for their role in primary and secondary immune responses.
Let's do one last review:
Match the lymphoid organ with its corresponding property/function.

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