Key messages

• The immune system functions to distinguish self and harmless non-self from harmful non-self and having identified the latter deploys effector mechanisms to eliminate it.

• Immune responses can be both antigen specific and antigen non-specific. Non-specific responses are usually associated with the induction of an antigen-specific response.

• Antigen-specific responses control the adaptive immune response; these are mainly mediated by B and T lymphocytes. The B-lymphocyte antigen receptor is immunoglobulin (antibody) which can be secreted to recruit effector mechanisms. T lymphocytes recognize antigen in the context of major histocompatibility complex (MHC) molecules using the T-cell receptor.

• Fragments of antigen recognized by T cells are produced by antigen processing. MHC class I molecules present intracellular peptides, including viral and tumor peptides, to CD8 T cells, resulting in a cytotoxic response. MHC class II molecules present extracellular antigens to CD4 T cells, resulting in B-cell antibody production or macrophage activation.

• Interaction between T and B cells allows T cells to activate B cells bearing antibody specific for the antigen recognized by the T cell.

• Tolerance to self is maintained through T cells. Self-reactive T cells are eliminated in the thymus. In the periphery two signals from a professional antigen-presenting cell are required to activate T cells; these signals are controlled to avoid activating self-reactive T cells.

• Antibody effector mechanisms include neutralizing free antigens, facilitating pathogen phagocytosis, and the activation of complement.

• Cellular effector mechanisms can destroy infected cells (cytotoxic T cells), engulf pathogens, and act against parasites (eosinophils).

• The normal functioning of the immune system is remarkable for its invisible nature.

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