Eicosanoids are products of arachidonic acid (AA), an essential fatty acid. They include prostaglandins (PGs), prostacyclin (PGI2), thromboxanes (TXs), leukotrienes (LTs) and hydroxy fatty acids. AA is derived enzymatically from membrane phospholipids and to a lesser extent the diet. Eicosanoids self-regulate every cell, including those synthesizing serotonin, norepinephrine and dopamine and those subserving immune function, such as T-cells, B-cells, natural killer cells, macrophages, monocytes and dendritic cells. There is objective evidence that prostaglandins regulate the physiology of the hypothalamic-pituitary-adrenal axis (HPA). Elucidation of the structure and metabolic pathways of eicosanoids galvanized researchers into illuminating their role in physiology, pathology and pharmacology. Striking contradictions arose: eicosanoids were shown to activate and suppress microorganisms, potentiate and suppress immunity and possess pro- and anticancer properties. As prostaglandins are the most heavily studied eicosanoids in the context of mood and immunity I will focus on them in this article. I will present evidence of the immunostimulating and antimicrobial properties of lithium and antidepressants and propose that these properties are linked to the antiprostaglandin actions of these compounds.