21. Lysozyme is present in

A. sweat
B. Cerebrospinal fluid (CSF)
C. urine
D. saliva

22. Macrophages are professional antigen-presenting cells. The protein molecule through which they present antigen in human is

A. actin
B. interleukin
C. HLA
D. CD8

23. The role of the M cell is to

A. trap virus in mucus and prevent entry
B. induce apoptosis in virus-infected small intestinal epithelial cells
C. pass bacteria or virus to underlying macrophages for processing and presentation of antigen
D. they use their cilia to propel mucus out of the small intestine

24. Which of the following statements is false regarding B cells?

A. B cells can undergo class switching and produce a different antibody type
B. B one marrow stem cells migrate to the thymus and develop into B cells
C. B cells synthesize antibodies and put them on their cell surface
D. B cells that recognize high concentrations of antigens (self) in the absence of other signals undergo apoptosis in the bone marrow