My Dictionary #1

1. Herd Immunity: When the majority of the population is immune to an infectious agent, thus significantly reducing the pathogen reservoir due to the low chance of a susceptible individual contacting an infected individual.

2. a) Endosteal Niche: Microenvironment in the bone marrow that fosters the development of hematopoietic stem cells and is postulated to associate specifically with self-renewing, long-term hematopoietic stem cells. The area directly surrounding the bone and in contact with bone-producing osteoblasts. 

    b) Vascular Niche: Microenvironment in the bone marrow that fosters the development of hematopoietic stem cells and is postulated to associate specifically with hematopoietic stem cells that have begun to differentiate into mature blood cells. The area directly surrounding the blood vessels and in contact with endothelial cells. 

3. Lamina Propia: Layer of loose connective tissue under the intestinal epithelium where immune cells are organized. The site of the GALT and part of the mucosal immune system. It contains a large number of B cells, plasma cells, activated T cells, and macrophages in loose clusters.