AP BIOLOGY

Detailed explanation-1: -Human leukocyte antigens (HLA) are a group of identification molecules located on the surface of all cells in a combination that is almost unique for each person, thereby enabling the body to distinguish self from nonself. This group of identification molecules is also called the major histocompatibility complex.

Detailed explanation-2: -The immune system recognizes invaders by their antigens, which are proteins on the surface of the invading cells (see Figure 1). Every cell or substance has its own specific antigens, and a person’s cells carry “self-antigens” that are unique to that individual.

Detailed explanation-3: -The immune system relies on antibody-antigen binding and the binding of antigens with receptors on B or T Cells to recognize non-self-antigens within the body.

Detailed explanation-4: -That is, the immune system must be able to distinguish what is nonself (foreign) from what is self. The immune system can make this distinction because all cells have identification molecules (antigens) on their surface. Microorganisms are recognized because the identification molecules on their surface are foreign.

Detailed explanation-5: -Antigens, small molecules, or peptides capable of eliciting an immune response are key elements in this process of distinguishing self from nonself. Antigens serve as labels that enable the immune system to distinguish between a normal interaction (self) and an encounter with a foreign threat (nonself).