The WBC that is considered “second line of defence” is:
Monocyte is considered as the second line of defence.
 
Phagocytosis by Macrophages.
Macrophages are the end-stage product of monocytes that enter the tissues from the blood. When activated by the immune system, they are much more powerful phagocytes than neutrophils, often capable of phagocytizing as many as 100 bacteria. They also have the ability to engulf much larger particles, even whole RBCs or, occasionally, malarial parasites, whereas neutrophils are not capable of phagocytizing particles much larger than bacteria. Also, after digesting particles, macrophages can extrude the residual products and often survive and function for many more months.
Reference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION ) page no 458
Neutrophil
Eosinophil
Basophil
Monocyte