Immune Responses: Primary and Secondary

Immune responses to antigens may be categorised as primary or secondary responses. The primary immune response to antigen occurs on the first occasion it is encountered. This response can take up to 14 days to resolve and leads to the generation of memory cells with a high specificity for the inducing antigen. The humoral response, mediated by B cells with the help of T cells, produces high-affinity and antigen-specific antibodies. In contrast, the CD8 T-cell response leads to the generation of large numbers of antigen-specific cells that are capable of directly killing infected cells. Antigen-specific CD4 T cells, which provide help to B cells, can also be expanded upon antigenic stimulation. The secondary response of B- and T cells following subsequent encounter with the same antigen is more rapid due to the activation of previously generated memory cells. This has quantitative and qualitative differences from the primary response. Key Concepts The innate immune system is the first line of defence against infectious agents. When this is breached, the adaptive immune system provides a more efficient response to clearing pathogens. The adaptive immune system has the capacity to ‘remember’ previous antigens, a process termed immunological memory. Antigen-specific T cells are selected during a primary immune response and expand to produce clones of T cells with high specificity for the activating antigen. In a B cell primary response to a thymus-dependent antigen, the immune system selects B cells with a high affinity and specificity for the antigen and these become memory cells. The selection of B cells with a high affinity for a given antigen occurs in the germinal centres of secondary lymphoid follicles and requires the enzyme activation-induced cytidine deaminase (AID) and interactions with other immune cells. The ability to change the isotype of antibody produced (class switching) by a B cell also occurs in germinal centres and requires AID. In a secondary response to the same antigen, memory cells are rapidly activated. This process is quicker and more effective than the primary response.