It makes the skin slightly acidic. B cells, T cells, Natural Killer cells. Opsonization of microbes by complement proteins and antibody facilitates phagocytosis. It inhibits the growth of some microbes. It increases the heart rate so that white blood cells, etc. are delivered to infection sites more rapidly.
Heat speeds up chemical reaction rates.
What is considered low grade in infants is much lower in adults. This is due to a baby's higher surface area to volume ratio. Basically, a baby has more surface area compared to her volume than an adult does. So, it's easier for heat to reach the skin and dissipate into the air. This binding induces the uninfected cells to synthesize antiviral proteins that interfere with or inhibit viral replication.