What type of antigens do MHC class II molecules primarily present?

MHC class II molecules primarily present extracellular antigens, which are typically derived from proteins that have been taken up by antigen-presenting cells through processes like endocytosis or phagocytosis. These antigens are processed within the antigen-presenting cell to produce peptides that are then loaded onto MHC class II molecules. Once presented on the surface of these cells, the peptide-MHC class II complexes can be recognized by CD4+ T helper cells, leading to an immune response.

This mechanism is crucial for the adaptive immune response, particularly in activating helper T cells that assist in orchestrating the body's response to pathogens, such as bacteria and foreign proteins. Knowing this, it’s clear why the other options do not fit the context: intracellular antigens are presented by MHC class I molecules, autoantigens are self-antigens potentially involved in autoimmune responses but do not specifically pertain to MHC class II presentation, and plant antigens do not relate directly to the classic functions of the human immune system.