Foreign molecules that elicit an immune response are called
a. Major histocompatibility complex (MHC) molecules.
b. Antibodies.
c. Histamines.
d. Antigens.

Antigens are foreign molecules, often proteins or polysaccharides, that trigger an immune response when detected by the body. They can originate from pathogens like bacteria and viruses, or from non-infectious sources such as pollen or transplanted tissues. The immune system recognizes these molecules as non-self, leading to the activation of immune cells and the production of antibodies.

The immune response is the body's defense mechanism against foreign invaders, including pathogens and antigens. It involves the recognition of antigens by immune cells, which then activate various pathways to eliminate the threat. This response can be innate (immediate and non-specific) or adaptive (specific and slower, involving memory cells for future protection).

The Major Histocompatibility Complex (MHC) is a set of molecules displayed on cell surfaces that play a crucial role in the immune system's ability to recognize foreign substances. MHC molecules present peptide fragments from proteins, including antigens, to T cells, facilitating the activation of the adaptive immune response. There are two main classes of MHC molecules: Class I, which presents to CD8+ T cells, and Class II, which presents to CD4+ T cells.