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Introduction to Pathology

Pathology is the “scientific study of disease”. It is the “scientific study of the molecular, cellular, tissue, or organ system response to injurious agents.” It “is the foundation of medical science and practice. Without pathology, the practice of medicine would be reduced to myths and folklore”.

Pathology serves as a “bridge” or “link” between the preclinical sciences (anatomy, physiology, etc.) and the courses in clinical medicine. It is the study of what is abnormal or wrong or disease…!

This brings us to the question what exactly is disease?

Disease is the “state in which an individual exhibits an anatomical, physiological, or biochemical deviation from the normal”. It may be defined as an abnormal  alteration of structure or function in any part of the body.

Classification of Diseases

1. Developmental – genetic, congenital.
2. Acquired:
a. Inflammatory – Trauma, infections, immune, etc.
b. Neoplastic – tumors cancers
c. Degenerative – aging.
d. Metabolic.
e. Iatrogenic: Drug induced.

What should we know About a Disease?

Regarding a disease process, we must be familiar with:

•    Definition of disease
•    Epidemiology – Where & When?
•    Etiology – What is the cause?
•    Pathogenesis – Evolution of disease
•    Morphology – Structural Changes
•    Functional consequences
•    Management
•    Prognosis
•    Prevention

Pathology focuses on 4 aspects of disease:

1. Etiology: Cause of disease.
2. Pathogenesis: Mechanisms of development of disease.
3. Morphology: The structural alterations induced in cell and tissues.
4. Functional Consequences: Functional results of the morphologic changes, as observed clinically.


Etiology is the “study of the cause of a disease”. Knowledge of etiology remains the backbone of disease diagnosis, understanding the nature of diseases and treatment of diseases. To understand the cause of disease, we need to be familiar with:

a. An etiologic agent is the factor (bacterium, virus, etc.) responsible for lesions or a disease state.
b. Predisposing Causes of Disease: Factors which make an individual more susceptible to a disease (damp weather, poor ventilation, etc.)
c. Exciting Causes of Disease: Factors which are directly responsible for a disease (hypoxia, chemical agents…. etc.).


Some diseases like diabetes and hypertension are multifactorial diseases involving:
a. Environmental agents:
•    Physical
•    Chemical
•    Nutritional
•    Infections
•    Immunological
•    Psychological

b. Genetic Factors:
•    Age
•    Genes


Pathogenesis is the sequence events in the response of the cells or tissues to the etiologic agent, from the initial stimulus to the ultimate expression of the disease,” from the time it is initiated to its final conclusion in recovery or death”
The core of the science of pathology is the study the pathogenesis of the disease.

Clinical Symptoms & Signs

Clinical symptoms are the patient’s complaints usually in patient’s own words. Clinical signs are seen only in the living individual. These are the functional evidence of disease which can be determined objectively or by the observer (fever, tenderness, increased respiratory rate, etc.).


Prognosis is the expected outcome of the disease, it is the clinician’s estimate of the severity and possible result of a disease.


Diagnosis is the the formal name(s) used to describe a patient’s disease. It is the process of identifying a disease based on the patient’s symptoms, the doctor’s findings, and the results of investigations and laboratory tests.

What do you need to make a diagnosis?

•    A system of classification that supplies the necessary names, definitions, and criteria
•    The means to ascertain the defining characteristics of a disease in the individual patient

Branches of Pathology

Pathology may be divided into following branches:
1.    General Pathology
2.    Systemic Pathology
3.    Gross Pathology
4.    Cellular Pathology
5.    Surgical Pathology
6.    Clinical Pathology
7.    Immunopathology

Subdivisions of Clinical Pathology:
•    Histopathology
•    Cytopathology
•    Haematology
•    Microbiology
•    Immunology
•    Chemical Pathology
•    Genetics
•    Toxicology
•    Forensic Pathology

Techniques Used in Pathology:

In pathology we use:

  • Light Microscopy
  • Histochemistry
  • Immunohistochemistry
  • Electron Microscopy
  • Cell Culture
  • Molecular Pathology


Morphology involves study of structural changes in disease. For example:

  • Tumor in a cancer.
  • Ulcer in an infection.
  • Atrophy in dementia.

Morphology may be gross or microscopic.

Some Terms:

Necropsy: Gross examination of the animal cadaver by systematic dissection in order to evaluate any abnormal changes (lesions) that may be present.
Autopsy: Synonymous to necropsy in human medicine
Biopsy: Removal and examination of tissue obtained from the living body
Euthanasia: Intentional putting to death of an individual with an incurable or painful disease by employing humane means

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