The molecules and cells of the immune system tend to fall into two categories — classical and non-classical. This month's issue contains several articles that focus on the non-classical category. For example, as described by Branch Moody and Steven Porcelli on page 11, CD1 molecules are MHC class-I-like molecules that present antigens to T cells — but, the antigens are glycolipids rather than peptides, the antigen-binding groove is different and CD1 molecules traffic through the cell using two separate, but parallel, pathways. The authors speculate that the endosomal pathway presents foreign glycolipids from exogenous sources preferentially, whereas self-lipids are processed mainly in non-endosomal compartments.
Macrophages are described classically as being activated by TH1 cytokines, but, as Siamon Gordon explains on page 23, this categorization is somewhat limited, because macrophages can also be activated in an 'alternative' manner by the TH2 cytokines IL-4 and IL-13.
