Collections of lymphoid tissue (MALT) that guard mucosal surfaces include all of the following except:
a. Appendix follicles
b. The tonsils
c. Peyer's patches
d. The thymus

Verified step by step guidance

1. Step 1: Understand what MALT (Mucosa-Associated Lymphoid Tissue) is. MALT consists of lymphoid tissues that protect mucosal surfaces such as those in the respiratory, digestive, and urogenital tracts.
2. Step 2: Identify the examples of MALT given in the options: Appendix follicles, tonsils, and Peyer's patches are all known collections of lymphoid tissue associated with mucosal surfaces.
3. Step 3: Recall the function and location of the thymus. The thymus is a primary lymphoid organ involved in T-cell maturation and is not associated with mucosal surfaces.
4. Step 4: Compare each option to the definition of MALT. Since the thymus is not part of the mucosa-associated lymphoid tissue, it does not belong in the list of MALT collections.
5. Step 5: Conclude that the correct answer is the option that is not part of MALT, which is the thymus.

Key Concepts

Mucosa-Associated Lymphoid Tissue (MALT)

MALT refers to collections of lymphoid tissue found in mucosal linings of the body, such as the digestive, respiratory, and urogenital tracts. These tissues play a key role in immune defense by detecting and responding to pathogens at mucosal surfaces.

Examples of MALT Structures

Common examples of MALT include the tonsils, Peyer's patches in the small intestine, and lymphoid follicles in the appendix. These structures contain immune cells that help protect mucosal surfaces from infection.

Thymus and Its Role

The thymus is a primary lymphoid organ responsible for T cell maturation, not a MALT structure. Unlike MALT, it is located in the chest and does not directly guard mucosal surfaces, making it distinct from other lymphoid tissues listed.