Haseeb Ullah

  Tolerance Tolerance in the immune system refers to a state where the immune system does not respond to specific antigens (or epitopes) even though it functions normally otherwise. This means that the immune system ignores certain substances as if they were not harmful. Typically, antigens that are present during embryonic development are identified as "self" and do not trigger an immune response. This is because self-reactive T-cell precursors are eliminated in the thymus during fetal development. Antigens encountered later, after the immune system has matured, are considered "NonSelf" and usually provoke an immune response. Although both T cells and B cells are involved in tolerance, T-cell tolerance is more crucial.   T Cell Tolerance Tell tolerance mainly occurs in the thymus during fetal development. This process, known as clonal deletion, involves the destruction of T cells that react against self-antigens (mainly self-major histocompatibility complex [

  Introduction   Immunity: Resistance or defense against harmful agents/substances causing disease.   Immune System: Cells and molecules defending the body against environmental pathogens.   Antigen: Substance stimulating an immune response.   Antibodies: Proteins reacting to antigens.   Self vs NonSelf   Self: Cells, fluids, molecules, and structures from the host.   NonSelf: External substances, living organisms (parasites, fungi), nonliving toxins, or host derived genetic rearrangements.   Types of Immunity   Innate or Natural Immunity: Primary, nonspecific defense present at birth, not altered by repeated exposure.   Acquired or Adaptive Immunity: Specific, acquired through contact with foreign substances, has memory improving response with each encounter.   Innate Immunity   First Line of Defense: Physical barriers like intact skin, mucous membranes.   Second Line of Defense: Cellular components like phagocytic cells, macrophages, NK cells, and hum