This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pathology
Lack of reaction to our own HLA ( human leukocyte antigen ) is known as? 
Select and explain. 
A. Autoimmunity, B. Clonal selection, C. Tolerance, D. Complement system
C: Tolerance
Answer: C: Tolerance 
Explanation: Immunologic tolerance is the phenomenon of unresponsiveness to an antigen-induced by exposure of lymphocytes to that antigen. Self-tolerance refers to lack of responsiveness to an individual's own antigens, and it underlies our ability to live in harmony with our cells and tissues. The mechanisms of self-tolerance can be broadly classified into two groups: central tolerance and peripheral tolerance Central Tolerance: In this process, immature self-reactive T and B lymphocyte clones that recognize self-antigens during their maturation in the central (or generative) lymphoid organs (the thymus for T cells and the bone marrow for B cells) are killed or rendered harmless. Several mechanisms silence potentially autoreactive T and B cells in peripheral tissues Anergy: Lymphocytes that recognize self-antigens may be rendered functionally unresponsive, a phenomenon called anergy. Suppression by regulatory T cells: A population of T cells called regulatory T cells functions to prevent immune reactions against self-antigens. Deletion by apoptosis. T cells that recognize self-antigens may receive signals that promote their death by apoptosis.. Robbins pathology 9e pg:214