A composition including a purified lipid cell-wall component or analog or derivative thereof and a suitable pharmaceutical carrier, medium, excipient or adjuvant is described. The composition is useful in prophylactic and therapeutic methods of treating a microbial infection in a subject, typically a mycobacterial infection such as tuberculosis, and immune disorders, inflammatory conditions and allergies in a subject, typically autoimmune diseases. It is also useful in diagnostic methods. The purified lipid cell-wall component is typically a purified mycolic acid or a mixture of purified mycolic acids from a bacterium which produces mycolic acids. The bacterium is from Mycobacterium, Corynebacterium, Nicardia or Rhodococcus.