Overview of Immune Responses

Abstract

The immune system comprises a network of humoral and cellular elements that recognizes self and non-self molecules or components through soluble and cell-associated receptors. The receptor-ligand interactions trigger molecular signaling that activate biochemical cascades and culminate with the production of effector mediators that regulate the cellular activity. The most common classification used to describe the immunological processes is the division in “innate” and “adaptive” immune responses. Innate immune responses occur immediately or very shortly after the first contact with any given aggressor agent that breaks the body’s physical, chemical and biological barriers. The elements of the innate immune system recognize conserved structures from these organisms and also endogenous molecules produced or released as a result of the invasion. This recognition activates cascades of soluble molecules such as those involved in the coagulation system and the complement system, as well as alarm cells, professional phagocytes and cytotoxic cells that constitute the innate responses. Meanwhile, lymphocytes—cells of adaptive immune responses—are activated and differentiate into effector cells. T cells are activated upon interaction with antigen-presenting cells and further differentiate into T helper, T cytotoxic or regulatory T cells that mediate cell-mediated adaptive responses by activating effector cells, directly killing of target cells or regulating immune responses. Meanwhile, B cells are activated by direct contact with the antigen (with or without help of T cells) and will mediate humoral adaptive immune responses through the release of antibodies. All these elements will be discussed in further details throughout this book in a context of immunometabolism.