Cells of the Immune System
Authors: Delves Peter J 
Published: August 2019

Abstract

The immune system includes a range of cell types with different roles in defending the body against infection. Most of these cells arise in the bone marrow, circulate in the blood and lymph and can migrate into solid tissues. B and T lymphocytes react to microbial antigens which they recognise using antigen‐specific receptors: the B‐cell receptor (BCR) and T‐cell receptor (TCR). Activated B cells develop into antibody‐secreting plasma cells. T cells can possess helper, regulatory or cytotoxic functions. Together, T‐cells and B‐cells constitute the adaptive (acquired) immune response and are able to generate memory cells. Innate lymphoid cells, including the cytotoxic natural killer (NK) cells, can also detect the presence of infection but do not do so in a highly antigen‐specific manner and do not exhibit classical immunological memory. Dendritic cells present antigens to T cells and follicular dendritic cells, an entirely different cell type, show antigen to B cells. Blood monocytes give rise to tissue macrophages that are phagocytic, as are circulating neutrophils. Eosinophils secrete toxic mediators. Blood basophils and tissue mast cells are important sources of inflammatory mediators. Other cells also contribute to immune and inflammatory responses, including endothelial cells, erythrocytes and platelets.

Key Concepts

A variety of cell types are important components of the immune system.

B and T lymphocytes (B and T cells) recombine immunoglobulin and T‐cell receptor genes, respectively, in order to generate highly antigen‐specific cell surface receptors. They are responsible for adaptive (acquired) immunity which exhibits immunological memory.

B cells recognise native (unprocessed) antigens using their surface immunoglobulins (the B‐cell receptor, BCR) and differentiate into plasma cells which release a secreted version of the same immunoglobulin (antibody).

Most T cells possess an αβ form of the T‐cell receptor (TCR) and recognise processed antigens (usually peptides) associated with MHC proteins expressed on the surface of antigen‐presenting cells. Different αβ TCR T cell subpopulations have helper, cytotoxic or regulatory functions. A minority of T‐cells, mostly located in mucosal and connective tissues, possess a γδ form of the TCR which does not require MHC proteins in order to recognise antigens.

All other cells of the immune system are classified as being innate in that they do not possess highly antigen‐specific receptors generated by genetic recombination and do not exhibit classical immunological memory.

Innate lymphoid cells (ILC) mirror the properties of the T‐cells of the adaptive response but lack the classical antigen‐specific receptors (BCR and TCR) created by genetic recombination. They include ILC1, ILC2 and ILC3 subpopulations with various helper activities, and natural killer (NK) cells which mediate cytotoxic activity against infected or malignant cells and therefore are analogous to cytotoxic T‐cells.

Monocytes are circulating blood cells that give rise to tissue macrophages with phagocytic and antigen‐presenting functions.

Dendritic cells are potent antigen‐presenting cells involved in the activation of T cells, whereas follicular dendritic cells show antibody‐associated antigens to B cells.

Granulocytes circulate in the blood, migrate into tissues, and include phagocytic neutrophils and eosinophils that secrete toxic mediators, and basophils that release inflammatory mediators; mast cells are tissue cells with similar properties to basophils.

A variety of other cell types contribute to the generation and regulation of immune and inflammatory responses, including endothelial cells, erythrocytes and platelets.