Immunology
Innate and adaptive immunity
Innate immunity (neutrophils, macrophages, NK cells, complement, TLRs) responds within hours without memory. Adaptive immunity is antigen-specific, clonal, and memory-forming. B cells produce antibody; CD4 helper T cells recognize antigen on MHC class II; CD8 cytotoxic T cells recognize MHC class I.
Antibody structure and classes
Fab determines specificity; Fc determines effector function. IgM is a pentamer and dominates the primary response; IgG is monomeric, crosses the placenta, and dominates secondary responses. IgA is secretory; IgE mediates type I hypersensitivity. Affinity is the strength of a single site; avidity is the combined strength of all sites.
Complement and hypersensitivity
Classical pathway begins with C1q binding IgG/IgM; alternative with microbial surfaces and properdin; lectin with mannose-binding lectin. Gell-Coombs: I IgE/mast cell, II antibody against tissue antigen (Coombs), III immune complex, IV delayed T-cell mediated.
Serology and traps
ANA is sensitive but not specific. Homogeneous pattern points to anti-dsDNA (specific for SLE; complement is low in active disease), while anti-histone suggests drug-induced lupus. Anti-CCP is more specific than rheumatoid factor. Complement is consumed in active immune-complex disease.
Trap: a positive ANA in a healthy patient is not diagnostic - order specific antibodies such as anti-dsDNA or anti-ENA.