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Introduction to the horse’s immune system

The immune system plays a crucial role in safeguarding the body against invading pathogens, such as viruses, bacteria, and foreign proteins. It is not a single organ, but rather a complex interaction of various components including organs, cells, and biochemical substances.

Key elements of the immune system

  • Leukocytes: White blood cells, encompassing lymphocytes, monocytes, and granulocytes.
  • Lymphocytes: T cells and B cells.
  • Monocytes: Give rise to macrophages, known as scavenger cells, which function in tissues.
  • Granulocytes: Divided into basophils, eosinophils, and neutrophils, contributing to cellular immune defence
  • NK cells: Combat tumour cells or virus-infected cells.
  • Antimicrobial peptides: small protein sequences that swiftly eliminate pathogens.
  • Plasma proteins: Diverse proteins involved in immune defence.
  • Complement system: Combination of antibodies and assorted plasma proteins that eliminate invading pathogens.
  • B cells: Responsible for antibody production.
  • Gamma/delta T cells: Identify stress proteins (e.g., from UV radiation) and eliminate affected cells.
  • Alpha/beta T cells: Classified into T helper cells and cytotoxic T cells.
  • T helper cells (CD4 cells): Initiate immune response.
  • Cytotoxic T cells (CD8 cells): Destroy infected cells.

Humoral and Cellular Immune Systems

Within the immune response, aside from different participants such as various cell types, a distinction is drawn between the innate and adaptive immune systems, as well as the humoral and cellular immune systems. The humoral immune system includes immune-active plasma proteins that circulate freely in the body, often through the bloodstream, without active mobility. Conversely, the cellular immune system encompasses all cells engaged in immune processes. These cells can exit blood vessels and migrate to target sites.

Innate and Adaptive Immune Systems

The innate immune system operates non-specifically against pathogens or harmful proteins, incapable of adaptation or change. For example, antimicrobial peptides present in sweat on the skin prevent pathogens from penetrating through the skin or mucous membranes. These mechanisms are “innate,” present from birth, and do not necessitate learning from prior infections.

The innate immune system responds within minutes upon recognising pathogens.

In addition to the innate immune system, an acquired immune system exists, which responds actively to invading pathogens. Unlike the innate system, this component is not fully developed at birth and gains its “training” through encounters with illnesses. Notably, this system features “antibodies” that selectively identify specific pathogenic proteins. Elaborate immune reactions are engaged in their production. The process begins with macrophages, also known as “scavenger cells,” engulfing foreign germs and presenting their proteins on their surface to the immune system. While macrophages belong to the non-specific, innate immune defence, they spark activation in the specific, acquired segment of the immune system.

T-helper cells scrutinise the presented protein fragments, initiating a cascade of reactions. Among these, B plasma cells are activated, leading to the generation of antibodies targeting the foreign protein. This process typically takes several days, culminating in the availability of antibodies after approximately one to two weeks. Once antibodies are present, the body can mount a swift and efficient defence against the pathogen. Simultaneously, memory B cells are produced alongside the antibodies. These cells are stored in the spleen, primed for immediate reactivation should the body reencounter the same pathogen. This mechanism empowers the body to respond promptly to subsequent infections, often displaying milder or no disease symptoms.

In addition to producing antibodies, B cells also yield toxins against pathogens, as well as activate the complement system, macrophages, or NK cells, all contributing to defensive functions. These cells also sensitise mast cells and granulocytes. T cells can be divided into two subtypes: alpha/beta T cells, encompassing T-helper cells (CD4 cells) and cytotoxic T cells (CD8 cells). T-helper cells engage in activating the antibody cascade, while cytotoxic T cells directly target and eliminate virus-infected or degenerate (tumour) cells, earning them the moniker of “killer cells”. A smaller subset comprises gamma/delta T cells, responsive to stress or heat shock proteins produced by cells (e.g., due to UV light-induced damage), with the capacity to neutralise them.

The acquired immune system operates with extraordinary specificity, yet it moves at a relatively slower pace compared to the rapid response of the innate immune system. The formation of antibodies can take several days, while a complete resolution of infection may require two to three weeks.

Horses at sunset
Once an infection has been recovered from, the body retains a “memory” so that it can fight it again more quickly. © Sanderstock / Adobe Stock

In practice, all parts of the immune system work together seamlessly

The initial counteraction against an infection is spearheaded by the innate, nonspecific defence. This, in turn, triggers the specific defence for targeted elimination of pathogens in a subsequent wave. Post-infection, the body retains a “memory” through memory B cells. In the event of a new infection, these cells promptly produce antibodies, invoking the specific defence. The duration of memory cell retention hinges on the pathogen type. Rapidly mutating viruses like influenza yield a shorter memory span, necessitating relearning with each infection due to their swift alteration. Conversely, slowly mutating bacteria like Clostridium tetani cultivate a more enduring memory, given their consistent appearance over time, incentivising the body to accumulate memory cells.

Furthermore, the immune system orchestrates repair processes post-injury, including the removal of cellular debris. It handles the elimination of malfunctioning endogenous cells like tumour or virus-infected cells, along with cells impaired by heat damage (e.g., sunburn). It also marks toxins for disposal. Hence, the immune system functions as both the guardian of health and, in multiple capacities, the body’s waste management system. It not only combats infections but also regulates internal processes, averts damage, and undertakes repairs.

More on this topic: 5 Tips for a Healthy Immune System or How Digestion Influences the Immune System