Phagocytic Cells

Phagocytic Cells

General

Ultimately, most pathogens are killed by phagocytes. Most cells have some phagocytic ability. The two most important cell types whose major function is phagocytosis are polymorphonuclear leukocytes (PMNs) and the monocyte-macrophage lineage cells (monocytes, macrophages (M), Kupffer cells, Langerhans cells, dendritic cells, and glial cells). They are sometimes known as professional phagocytes.

Phagocytes face two major problems: they must come into contact with the potential pathogen and they must recognize the pathogen as something that must be destroyed.

Surface Receptors on Phagocytes

M PMN

CR 1, 2, & 3 (receptors that bind with different affinity to C3b, C3bi, and C4b) X X

FcR I (receptor for IgG 1 & 3) X

MHC class I (receptor for CD8) X X

MHC class II (receptor for CD4) X

Chemotaxis: The first problem is solved by chemotactic factors. Phagocytes have receptors scattered uniformly over their surfaces. The phagocytes move in the direction of the part of the surface that has the most receptors filled; i.e., toward the higher concentration of factor. The most important chemotactic factors are peptides that start with formyl-methionine (which are made by bacteria but not animals), C5a, and C3a (about 1% of the activity of C5a).

Opsonization: Phagocytes have some intrinsic ability to recognize many bacteria and damaged tissue as needing to be removed. However, this process is significantly enhanced by factors that label foreign material for the phagocytes (opsonins). One requirement for a substance to serve as an opsonin is for the phagocyte to have a membrane receptor that binds to the opsonin.

As discussed earlier in the chapter, C3b is an excellent opsonin because PMNs and macrophages have receptors for C3b (and some of its breakdown products). The other major opsonin is immunoglobulin G (discussed in chapter 3).