Response of the innate and adaptive immune systems to circulating antigens, tissue antigens, and antigen-antibody complexes (literature review)

The innate and adaptive immune systems respond differently to circulating antigens, tissue antigens, and antigen-antibody complexes due to their distinct mechanisms and roles in immunity.

Innate Immune System Response

 •    Circulating Antigens: The innate immune system provides an immediate, non-specific response. It uses pattern recognition receptors (PRRs) to identify common pathogen-associated molecular patterns (PAMPs) on circulating antigens. Key components include phagocytes (e.g., macrophages, neutrophils) that engulf pathogens, and the complement system, which enhances phagocytosis and can directly lyse pathogens.

 •    Tissue Antigens: When antigens are present in tissues, the innate immune system responds by recruiting immune cells to the site of infection or injury. This involves the release of cytokines and chemokines that promote inflammation and attract cells like neutrophils and macrophages to the affected area.

 •    Antigen-Antibody Complexes: These complexes can activate the complement system, leading to opsonization and enhanced phagocytosis by innate immune cells. Fc receptors on innate cells bind to antibodies in the complexes, triggering phagocytosis or cell activation.

Adaptive Immune System Response

 •    Circulating Antigens: The adaptive immune response is slower but more specific. Circulating antigens are typically captured by antigen-presenting cells (APCs) like dendritic cells, which process and present them to T cells in lymphoid tissues. This initiates a specific T cell response, leading to clonal expansion and differentiation into effector T cells.

 •    Tissue Antigens: Adaptive responses to tissue antigens involve effector T cells migrating to the site of infection or inflammation. CD8+ cytotoxic T cells can directly kill infected cells, while CD4+ helper T cells assist by activating other immune cells such as macrophages.

 •    Antigen-Antibody Complexes: These complexes can also influence adaptive immunity by enhancing antigen presentation. B cells can bind these complexes through their B-cell receptors (BCRs), leading to activation and differentiation into antibody-secreting plasma cells. Additionally, immune complexes can modulate antibody production through feedback mechanisms involving Fc receptors on B cells.

Synergy Between Innate and Adaptive Responses

The innate immune system often acts as a first line of defense, providing immediate responses that help contain infections until the adaptive immune system is activated. Information from the innate response is responsible  for activating adaptive immunity; for example, cytokines produced by innate immune cells influence the type of adaptive response generated. Adaptive responses then refine and amplify the initial defense provided by innate immunity, often involving memory formation for quicker responses upon re-exposure to the same antigen.