How do leukocytes recognize microbial structures

Some leukocytes in the innate immune system are able to recognise microbial
structures directly.

Just wondering how?

Antigen's on the surface of bacteria. The body can recognize self from non-self and those antigens let them know that bacteria are foreign. So you can get an innate immune response like phagocytosis by neutrophils or macrophages or activation of the complement system.