Pathogens that get through barrier defenses are eaten by phagocytic cells, which include macrophages and dendritic cells invertebrates. Natural killer cells, which may destroy virus-infected cells, are another type of cellular defense.

Pathogens are also inhibited by complement system proteins, interferons, and other antimicrobial peptides. Histamine and other substances produced at the location of the injury increase c in the inflammatory response.

Pathogen-specific identification is provided by receptors in adaptive immunity.

Adaptive immunity is based on two types of lymphocytes that develop from bone marrow stem cells: B cells and T cells.

Lymphocytes contain antigen receptors on their cell surfaces that recognize foreign substances (antigens). Although all receptor proteins on a single B or T cell are the same, there are millions of B and T cells in the body that differ in the foreign molecules recognized by their receptors.

When a pathogen infects a person, B and T lymphocytes that are specific for the pathogen become activated. Some T cells aid other lymphocytes, while others destroy contaminated host cells. B cells, also known as plasma cells, generate soluble proteins known as antibodies, which attach to foreign molecules.

The creation of cell diversity, self-tolerance, proliferation, and immunological memory are the four key features of B and T cell development. Both proliferation and memory are reliant on clonal selection, as seen here for B cells:

Adaptive immunity protects the body against infection of bodily fluids and cells.

Helper T cells interact with antigen fragments presented on the surface of antigen-presenting cells such as dendritic cells, macrophages, and B cells via class II MHC molecules. Helper T cells that have been activated release cytokines that encourage other lymphocytes. Activated cytotoxic T cells in the cell-mediated immune response destroy infected cells. Antibodies aid in the elimination of antigens in the humoral immune response by increasing phagocytosis and complement-mediated lysis.

Active immunity develops as a result of infection or vaccination. Antibody transfer in passive immunity gives rapid, short-term protection.

Immune system dysfunction can cause or exacerbate illness.

The interaction of antibodies and allergens in allergies, such as hay fever, causes immune cells to produce histamine and other mediators that induce vascular alterations and allergic symptoms.

Autoimmune illnesses, such as multiple sclerosis, can develop as a result of a loss of self-tolerance. Inborn immunodeficiencies are caused by abnormalities in innate, humoral, or cell-mediated responses. AIDS is a viral infection that causes acquired immunodeficiency.

Some viruses can defy immune responses by antigenic variety, latency, and direct attack on the immune system. HIV infection kills helper T cells, making the patient susceptible to infection.

Pathogens such as bacteria and fungi are presented in the attached image, which has an overview of animal immunity. Innate immunity offers a primary defense in all animals and sets the stage for adaptive immunity in vertebrates.