The innate and active immune systems work together to protect the body against pathogens. In the first instance of a pathogen being identified, innate immunity has the fastest response time.
During this response, phagocytic cells (such as macrophages, monocytes, dendritic cells, …) may identify the foreign (non-self) antigens on the surface of the pathogen, inducing ingestion and digestion by lysozymes. Where a pathogen is not detected and resolved by phagocytic cells, the complement system can be used. Complement uses soluble factors that are activated by different methods. The MB-lectin pathway involves the binding of lectin, a bacterial sugar, to mannose, leading to C3 convertase being activated and ultimately C9 polymerisation to form the membrane attack complex (MAC). Complement factors produced through the cascade, especially C3a and C5a, induce chemotaxis in phagocytic and other immune cells (including T cells) with inflammatory mediators, helping activate the adaptive immune system.
The adaptive immune system incorporates both parts of humoral and cell-mediated immunity, targeting cytotoxic T cells and antibodies to the foreign antigen. Helper T cells present the foreign antigen on their cell surface, transporting through the lymph to lymphoid organs (thymus, lymph nodes, …) to activate B cells. A specific B cell able to produce a specific and complementary antibody is activated, undergoing clonal expansion to produce many plasma cells, secreting large quantities of antibody. As this immune response continues, the B cells undergo affinity maturation – this helps produce more specific antibody (by somatic hypermutation) to make the humoral response more effective.
The binding of an antibody/antibodies with 2 Fc domains allows C1q to bind – this catalyses the start of the classical complement pathway to support the innate immune system.
Natural killer (NK) cells can be involved in both the innate and adaptive responses. Suppression / absence of MHC class I proteins on the surface of a cell, viral infection (or some other problem) may have occurred. This induces the release of perforin and granzymes to kill the cell by apoptosis. Where antibody is being produced in the adaptive immune system, antibody-dependent cell mediated cytotoxicity (ADCC) may be used. Here, the NK cell recognises the Fc domain of the antibody, allowing it to target labelled (opsonised) cells.
On the first exposure to a pathogen, the innate immune system will be dominant in response, due to its quick response. As time continues, the adaptive immune system will be activated and begin the production of antibody, allowing a strong and specific response. Subsequent exposure to the same antigen will activate the adaptive immune system more quickly, giving a more effective response with higher levels of IgG synthesised in less time.