Development of Immunocompetence

Cell-mediated immunity in a fetus begins at the third month of growth, and active antibody-mediated immunity follows about one month later.

The first cells to migrate from the fetal thymus mainly inhabit the skin, the lining of the mouth, and the digestive tract. These cells, including skin-based dendritic cells that act as antigen-presenting cells, are crucial for T cell activation.

T cells that later depart from the thymus populate the lymphoid organs across the body.

The fetal liver and bone marrow produce early B cells with IgM antibodies on their plasma membranes.

Typically, maternal IgG antibodies provide ample passive immunity, though by the fourth month, limited fetal IgM production can occur upon exposure to high levels of pathogens.

After birth, infants receive IgA antibodies through breast milk. The maternal IgG supply is cut off and declines in the first two months.

From birth to age 12, children encounter new antigens, gradually building their memory B cells, T cell counts, and antibody levels to those typical in adults.