The lamina propria is a constituent of the moist linings known as mucous membranes or mucosa, which line various tubes in the body (such as the respiratory tract, the gastrointestinal tract, and the urogenital tract).
The lamina propria (more correctly lamina propria mucosæ) is a thin layer of loose connective tissue which lies beneath the epithelium and together with the epithelium constitutes the mucosa. As its Latin name indicates it is a characteristic component of the mucosa, "the mucosa's own special layer". Thus the term mucosa or mucous membrane always refers to the combination of the epithelium plus the lamina propria.
The lamina propria contains capillaries and a central lacteal (lymph vessel) in the small intestine, as well as lymphoid tissue. Lamina propria also contains glands with the ducts opening on to the mucosal epithelium, that secrete mucus and serous secretions. The lamina propria is also rich in immune cells known as lymphocytes. A majority of these cells are IgA-secreting B cells.