Despite comprising only 10% of the total number of immune cells, macrophages contribute nearly 50% of the total cellular mass of the immune system due to their large size1. This disproportionate contribution to immune mass highlights the significant role macrophages play in the body's defense mechanisms. These versatile cells are found in most tissues and perform crucial functions such as phagocytosis of foreign agents, elimination of apoptotic cells, and recycling of nutrients2.

Macrophages exhibit remarkable plasticity, capable of adopting different phenotypes in response to environmental cues. They can polarize into pro-inflammatory M1 macrophages, which initiate immune responses against pathogens, or anti-inflammatory M2 macrophages, which promote tissue repair and resolution of inflammation34. This functional diversity, combined with their substantial mass contribution, underscores the importance of macrophages in maintaining immune homeostasis and orchestrating both innate and adaptive immune responses.

The weight of individual immune cells varies significantly, reflecting their diverse functions and sizes within the immune system. Here's a breakdown of the average weights for key immune cell types:

• Neutrophils: Approximately 7.5 × 10^-12 g (7.5 picograms) per cell1

• Lymphocytes: Around 3.5 × 10^-12 g (3.5 picograms) per cell1

• Macrophages: Significantly larger, weighing about 5 × 10^-10 g (500 picograms) per cell1

• Eosinophils: Roughly 8 × 10^-12 g (8 picograms) per cell1

• Basophils: Approximately 9 × 10^-12 g (9 picograms) per cell1

These weight differences contribute to the overall mass distribution of the immune system, with macrophages accounting for nearly 50% of the total cellular mass despite comprising only 10% of immune cells23. This disparity in individual cell weights explains why certain cell types, though fewer in number, can contribute significantly to the immune system's total mass.

The bone marrow serves as a crucial reservoir for various immune cells, playing a vital role in both innate and adaptive immunity. It houses a significant population of mature neutrophils, forming the bone marrow reserve, which can be rapidly mobilized during inflammatory episodes or infections1. This reserve allows for a dramatic increase in circulating neutrophil numbers when needed.

Additionally, the bone marrow contains a substantial population of memory T cells, particularly central memory CD8+ T cells (TCMs)2. These TCMs are efficiently recruited to and retained in the bone marrow, where they can mount potent antigen-specific recall responses for extended periods2. The bone marrow also harbors B lymphocytes and plasma cells, contributing to long-term immune memory and antibody production34. This diverse immune cell reservoir in the bone marrow ensures a rapid and effective response to various immunological challenges, highlighting its importance in maintaining overall immune system readiness.

Lymphocytes and neutrophils, two crucial components of the immune system, exhibit distinct distribution patterns within the body. Lymphocytes, comprising about 40% of total immune cells, are primarily concentrated in lymphoid tissues such as lymph nodes and the spleen1. These cells, which include T cells, B cells, and natural killer cells, circulate between blood and lymph, continuously patrolling for potential threats2.

Neutrophils, on the other hand, account for a similar proportion of immune cells as lymphocytes but have a different distribution. The majority of neutrophils reside in the bone marrow, with only a fraction circulating in the blood under normal conditions1. As the first responders to infection or injury, neutrophils can quickly mobilize from the bone marrow and migrate to sites of inflammation3. This strategic distribution allows for rapid deployment of neutrophils while maintaining a reserve pool, ensuring an efficient immune response when needed.