Veterinary Immunology and Serology

BACTERIAL ANTIGENS

• There are two main groups
  • Soluble antigens: Some soluble substances produced by the bacteria, which are excreted into the environment. For example, toxins, enzymes etc.
  • Cellular antigens: They are the structural units of bacterial cell. Common bacterial antigens are:
    • Somatic (O) antigen: In gram negative bacteria (Salmonella, E. coli, Brucella etc.), somatic antigens are composed of lipopolysaccharide (LPS)–protein complex, which are good antigen and produce good immune response. But O antigens are highly variable (LPS consists of highly variable Oligosaccharides) and thus immunity against one ‘O” antigen will not confer immunity against bacteria bearing other ‘O” antigens. But a common core antigen (under laying core polysaccharide) has a potential use as vaccine.
    • Capsular (K) antigen: A variety of bacterial species have capsule (e.g. Bacillus anthracis, E. coli, Salmonella spp. etc) which is antigenic. Capsule commonly consists of polysaccharides (e.g. K antigen of E.coli) but some are composed of polypeptides (e.g. Poly-D-glutamic acid in case of B.anthracis).
    • Flagellar (H) antigen: Motile bacteria have flagella (e.g. Salmonella spp., E.coli, Proteus spp.). These Flagella is composed of protein (flagellin), which is antigenic.
    • Fimbrial (F ) antigen: Fimbrial or Pili antigen are present on the surface of bacteria .
    • Spore antigen: Bacterial spores (e.g. Bacillus spp., Clostridium spp. etc.,) especially the exosporidium is antigenic.
    • Other significant bacterial antigens include the porins, heat shock proteins, the exotoxins etc.