Major Categories of Phagocytes

Introduction

Chemotherapy

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The Stages of HIV Infection

The Infectiveness and Pathogenicity of HIV

The Structure of HIV

The Origin of AIDS

Acquired Immunodeficiency Syndrome (AIDS)

Acquired Immunodeficiencies

Congenital Immunodeficiencies

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Mechanisms of Phagocytic Recognition, Engulfment, and Killing

Major Categories of Phagocytes

Lecture 3. The immune recognition of various foreign substances. Secondary Immunodeficiencies

Plan:

Phagocytosis: Partner to Inflammation and Immunity

By any standard, a phagocyte represents an impressive piece of living machinery, meandering through the tissues to seek, capture, and destroy a target. The general activities of phagocytes are

1. to survey the tissue compartments and discover microbes, particulate matter (dust, carbon particles, antigen-antibody complexes), and injured or dead cells;

2. to ingest and eliminate these materials; and

3. to extract immunogenic information (antigens) from foreign matter.

It is generally accepted that all cells have some capacity to engulf materials, but professional phagocytes do it for a living. The three main types of phagocytes are neutrophils, monocytes, and macrophages.

Neutrophils and Eosinophils As previously stated, neutrophils are general-purpose phagocytes that react early in the inflammatory response to bacteria and other foreign materials and to damaged tissue. A common sign of bacterial infection is a high neutrophil count in the blood (neutrophilia), and neutrophils are also a primary component of pus. Eosinophils are attracted to sites of parasitic infections and antigen-antibody reactions, though they play only a minor phagocytic role.

Macrophages: Large-Tissue Phagocytes After emigrating out of the bloodstream into the tissues, monocytes are transformed by various inflammatory mediators into macrophages. This process is marked by an increase in size and by enhanced development of lysosomes and other organelles (figure 1). At one time, macrophages were classified as either fixed (adherent to tissue) or wandering, but this terminology can be misleading. All macrophages retain the capacity to move about. Whether they reside in a specific organ or wander depends upon their stage of development and the immune stimuli they receive. Specialized macrophages called histiocytes migrate to a certain tissue and remain there during their life span. Examples are alveolar (lung) macrophages; Kupffer cells in the liver; Langerhans cells in the skin; and macrophages in the spleen, lymph nodes, bone marrow, kidney, bone, and brain. Other macrophages do not reside permanently in a particular tissue and drift nomadically throughout the RES. Not only are macrophages dynamic scavengers, but they also process foreign substances and prepare them for reactions with certain lymphocytes.

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