Which of what? I need more information to decipher that.
 
        
             
        
        
        
Answer:
Ethylene
Explanation:
Ethylene is a hormone required to trigger fruit ripening, and it can be blocked by using synthetic compounds, such as 1-methyl-cyclo-propene (1-MCP).
 
        
             
        
        
        
Answer:
peptide antigens by MHC class II molecules
Explanation:
Helper T cells are activated by peptide antigens by MHC class II molecules. These antigens express on the surface of antigen-presenting cells (APCs). After activation, the T helper cells divide and secrete cytokines  to assist the immune response.
 
        
             
        
        
        
Protozoans, bacteria, fungi... viruses are not organisms