Deer antler velvet may help to increase macrophage activity. Macrophages are a type of white blood cell (immunity cell) that digests cellular debris, foreign substances, cancer cells, and other microorganisms. Macrophages help to stimulate defense mechanisms in the body by recruiting immune cells such as lymphocytes. With this increased number of white blood cells, your immunity system becomes strengthened and will have a better chance at fighting off a sickness or infection.
A systemic review on human interventions makes note of a study conducted on patients of osteoarthritis (Edelman et al. 2000; cannot be located online) which found improvements in joint pain symptoms relative to baseline in the Velvet Antler group and not placebo, although a lack of information on blinding and randomization precludes results that can be drawn from this study.