The Immune response is the body's response caused by its immune system being activated by antigens. The immune response can include immunity to pathogenic microorganisms and its products, allergies, graft rejections, as well as autoimmunity to self-antigens.

In this process the main cells involved are T cells and B cells (sub-types of lymphocytes), and macrophages (a type of leucocyte or white blood cell). These cells produce lymphokines that influence the other host cells' activities. B cells, when activated by helper T cells undergo clonal expansion.

B cells differentiate into plasma cells, which are short lived and secrete antibodies, and memory B cells, which are long lived and produce a fast, remembered response when exposed to the same infection in the future. B cells mature to produce immunoglobulins (also known as antibodies), that react with antigens.

Innate response is evolutionary more conserved than adaptive immune response. It is the first line of defense when it comes to defending an organism from an foreign invader.

Foreign invaders include bacteria, viruses, and parasites. Innate immunity responses are not specific to particular pathogen. They are rather limited to conserved features amongst pathogens.

An immune response can usually be described generally as "The reaction of the host's immune system to antigen in an invading (infecting) pathogenic organism, or to foreign protein, as in transplanted organs or tissues.

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