Table 6:

Factors responsible for the relative weakness of the immune system in infants

Cells	Function
Macrophage	Defective secretion of pro-inflammatory cytokines IL-12 and TNF-α
Natural killer (NK) cells	Three times weaker cytotoxic function than in adults
Complement	Lower complement levels, and functionally weaker than in adults
Natural antibodies	Few present until 3–6 month of age
T helper cell response	High anti-inflammatory cytokine profile (IL-4, IL-5 and IL-10), Th2>Th1
Cytotoxic T (Tc) cells	Defective secretion of IL-12
T regulatory cells (Tregs)/Th17	Absent Th17, and more abundant Tregs, that are responsible for T cell suppression
B cells	Defective CD40 expression, and immature B cell response