Immunity

The essentials

The overall ability of the host to fight disease-causing organisms, conferred by the immune system, is called immunity.

Innate immunity is non-specific and present at birth. It has four types of barriers:

• Physical barriers: skin, and the mucus coating of the respiratory, gastrointestinal and urogenital tracts.
• Physiological barriers: acid in the stomach, saliva in the mouth and tears from the eyes.
• Cellular barriers: leucocytes such as polymorpho-nuclear leucocytes (PMNL, neutrophils) and monocytes, natural killer lymphocytes, and macrophages that phagocytose microbes.
• Cytokine barriers: virus-infected cells secrete interferons, which protect non-infected cells from further viral infection.

Acquired immunity is pathogen-specific and has memory. The first encounter produces a low-intensity primary response; a later encounter with the same pathogen produces a highly intensified secondary (anamnestic) response. It is carried out by two kinds of lymphocytes. B-lymphocytes produce proteins called antibodies into the blood (humoral immune response). T-lymphocytes do not secrete antibodies but help B cells and mediate cell-mediated immunity (CMI), which is responsible for graft rejection.

Each antibody molecule has four peptide chains, two small light chains and two longer heavy chains, so it is written as H₂L₂. Types include IgA, IgM, IgE and IgG.

Active immunity: the host produces antibodies after exposure to antigens (by infection or vaccination); it is slow but long-lasting. Passive immunity: ready-made antibodies are given, such as IgA in colostrum, IgG passing through the placenta, or anti-tetanus or anti-venom injections; it is quick but short-lived.

Lymphoid organs: primary lymphoid organs (bone marrow and thymus) are where lymphocytes are produced and mature. Secondary lymphoid organs (spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and appendix) are where lymphocytes interact with antigens. Mucosa-associated lymphoid tissue (MALT) forms about 50% of the lymphoid tissue in the body.

Common mistakes

• T cells do not produce antibodies.