In pedigree B, how can unaffected parents have an affected child?

The unaffected mother, who is marrying in, does not carry an allele for the disease; so the affected child inherits an allele only from the affected father. No child could be affected by a single autosomal recessive allele, or X-linked recessive allele, so the trait is dominant

The childs mother could be a carrier without knowing it which would create the couple to have a child thats affected.

Explanation:

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.