Notes on Blood Formed Elements, Phagocytosis, and Inflammation

Major formed elements: red blood cells (RBCs), platelets, and leukocytes (white blood cells).
The figure tracks the steps that formed elements must move through as they develop and function.
The formation starts from pluripotent hematopoietic stem cells, which are multipotent or pluripotent and can differentiate into all blood cell types.
Hematopoietic stem cells differentiate into two main lineages: a myeloid stem cell pathway or a lymphoid stem cell pathway.

Myeloid stem cell pathway can differentiate into an erythrocyte, a mast cell, or eventually a macrophage.
Lymphoid stem cell pathway can differentiate into lymphocytes such as B cells, T cells, and natural killer (NK) cells.
Granulocytes are characterized by granules visible in the cytoplasm; examples include basophils, neutrophils, and eosinophils.
Agranulocytes are cells that lack granules in their cytoplasm; examples include monocytes (which differentiate into macrophages or dendritic cells) and lymphocytes (which include B cells, T cells, and plasma cells).

Granulocytes:
- Basophils, neutrophils, and eosinophils contain cytoplasmic granules.
- The contents of the granules confer specific abilities to each cell.
- Eosinophils are described as being full of histamine and related mediators; degranulation helps target worms and other parasites (protozoans and helminths) in the intestines.
- Neutrophils also contain granules; when activated, they release chemical mediators (such as nitric oxide and other enzymes) to help break down pathogens in the extracellular space.
Agranulocytes:
- Lack granules in the cytoplasm.
- Monocytes differentiate into macrophages or dendritic cells, which are active phagocytic cells.
- Lymphocytes (T cells, B cells, and NK cells) are agranular.

Neutrophils are polymorphonuclear neutrophils (PMNs) named for their nucleus, which takes on various shapes and stains dark purple.
They are phagocytic and actively participate in infectious defense by engulfing pathogens and releasing granule contents.
Granules contain toxic molecules and enzymes that help destroy pathogens when released.

Monocytes leave the bloodstream and differentiate in tissues into macrophages or dendritic cells.
Macrophages: large white blood cells, highly phagocytic, and important antigen-presenting cells (APCs).
Dendritic cells: reside mainly in the skin and mucosal membranes, survey tissues for invaders, and are highly phagocytic, rapidly engulfing potential pathogens.
Both macrophages and dendritic cells serve as essential bridges between innate and adaptive immunity through phagocytosis and antigen presentation.

Macrophages phagocytose a pathogen, digest it, and present its antigens to cells of the adaptive immune response (B cells and T cells).
This antigen presentation links innate recognition to the activation of the adaptive immune system.

The process is commonly broken down into four main phases: chemotaxis, adherence, ingestion, and digestion, followed by excretion of waste.
Step 1: Chemotaxis – phagocytic cells are stimulated by chemical signals and migrate to the pathogen or injury site.
Step 2: Adherence – phagocytes recognize pathogen-associated molecular patterns (PAMPs) on pathogens via receptors (e.g., Toll-like receptors) and attach to the pathogen.
Step 3: Ingestion – a phagosome forms around the pathogen.
Step 4: Digestion – the phagosome fuses with a lysosome to form a phagolysosome; digestive enzymes break down the pathogen.
Step 5: Excretion – residual material is expelled from the cell.
PAMPs are conserved molecular patterns on pathogens recognized by phagocytes; Toll-like receptors detect these patterns to trigger phagocytosis.

Inflammation is a local immune response presenting with heat, redness, edema (swelling), pain, and loss of function.
Immediate vascular responses:
- Brief vasoconstriction helps limit blood loss and assists in clot formation.
- Local cells release vasoactive chemicals, such as prostaglandins and histamine, that dilate local blood vessels and increase blood flow to the area.
- Endothelial cells contract, creating gaps between them and increasing capillary permeability.
- Fluids and proteins pass from the blood into the tissue, contributing to edema.
Chemotaxis during inflammation: circulating neutrophils move from the blood to the injury site guided by chemoattractants.
Diapedesis: neutrophils squeeze through endothelial gaps and migrate toward the site of injury following a chemotactic gradient.
At the injury site: neutrophils encounter bacteria, engulf them, and digest them via phagocytosis.
Tissue repair: growth factors stimulate local fibroblasts to divide and secrete collagen, reinforcing the wound.

Macrophages and dendritic cells provide a critical link between innate recognition and adaptive immunity via antigen presentation.
Neutrophils are among the first responders in acute inflammation, equipped with granules to combat pathogens.
Dendritic cells act as sentinels in skin and mucosa, presenting antigens and activating T cells in lymph nodes to drive adaptive responses.
The inflammatory cascade coordinates vascular changes and immune cell recruitment to contain infection and promote healing.