The following statement(s) is/are true concerning cellular defense mechanisms

b. Macrophages may become activated and secrete cytokines 

d. Polymorphonucleocytes (PMNS) are normally present in only small numbers within the tissue and enter an area of infection through diapedesis

A wide variety of cell types serve to provide host defense at several levels. Macrophages act as the initial antigen processing cell that serves to present antigen to help T cells, thus initiating the immune response. Macrophages, however, are pluripotent cells that, in the process of engulfing and processing antigen, may become activated. Activated macrophages secrete a variety of cytokines. Macrophages also act as phagocytic cells in the tissues and within the bloodstream, and because of their resident nature in many tissues, also represent the first line of host defenses in many areas of the body, even before activation. PMNS are present within the bloodstream, but only in small numbers within the tissue, and enter an area of infection through diapedesis after chemotactic stimuli are excluded by macrophages, bacterial breakdown products, and complement activation.